One of the proof texts Jehovah's Witnesses use to show that Jesus was created is Colossians 1:15 which says that Jesus is "the firstborn of all creation." There are two reasons they think this phrase entails that Jesus was created. The first reason is because of the use of the word, "firstborn," which they take to literally mean Jesus was the first one among many to be born, i.e. created. The second reason is because of the grammatical construct of the phrase. To say that Jesus is the firstborn of all creation is to say that there's a category--creation--and the firstborn is a member of that category.
The first reason
I grant that the usual meaning of firstborn is the first child born among other siblings. There are lots of examples of firstborn being used that way. But that is not always how firstborn is used.
In Exodus 4:21-23, God tells Moses to go to Pharaoh and say, "Thus says the LORD, 'Israel is My son, My firstborn. So I said to you, "Let My son go that he may serve Me"; but you have refused to let him go. Behold, I will kill your son, your firstborn.'" In the context, it's clear that "Israel" refers to the nation of Israel, not to Isaac's son (Jacob was renamed Israel after a wrestling match with God). Isaac's firstborn son was Esau, but through trickery, Jacob robbed Esau of his birthright. Some would argue that Jacob then became the firstborn in which case firstborn refers to inheritance and/or preeminence, but Jacob/Israel was not literally Isaac's first child.
So in what sense is Israel--the nation--God's firstborn? I suspect the nation is the firstborn in a similar sense that Jacob became the firstborn. Israel was God's chosen people who inherited the promises God made to Abraham just as Jacob did. Israel wasn't the first nation that came to be. You might say Israel was God's first specially chosen nation, but there are no other nations God ever chose in the same sense, so they are not the firstborn among any peers, so to speak. You might say they have some preeminence over other nations, though, since they have a special relationship with God.
A mistake I think Jehovah's Witnesses make is putting too much emphasis on etymology. Etymology can only tell what a word originally meant, but words change meaning over time as they are used in different ways. They make this same mistake when arguing that Jesus died on a simple upright pole without a crossbeam merely because of the etymology of the word stauros. We still refer to transmission lines as "telephone poles" even though they often have crossbeams and are no longer used to hold telephone wires. We refer to fixing a computer program as "debugging" even though we no longer have to literally remove bugs from switches.
In the same way, firstborn came to mean something other than the first child born to its parents. One example is when Jacob blesses his two grandsons, Ephraim and Manasseh. Manasseh was born first, and Ephraim was born second (Genesis 41:51-52). When Joseph brought his sons to his father, Jacob (aka Israel), to bless them, Jacob crossed his hands and put his right hand on the younger son and his left hand on the older. Joseph tried to correct Jacob because apparently the right hand was supposed to be laid on the firstborn to confer a special blessing on him. But Jacob insisted on keeping his right hand on the younger son, saying that Ephraim would be greater than Manasseh. "Thus," the passage goes, "he put Ephraim before Manasseh" (Genesis 48:8-22). Consequently, Ephraim was called the firstborn (Jeremiah 31:9) even though he was born second.
David was also called the firstborn (Psalm 89:27) even though he was not the first child born to his father, Jesse, nor the first king of Israel. The only sense in which David could have been a firstborn was in the sense of his preeminence. The context bears this out as well. It says, "I also shall make him my firstborn, the highest of the kings of the earth" (Psalm 89:27). That's what firstborn means in this context--the highest of the kings of the earth.
Firstborn is a messianic title because it is used of David in a messianic sense in Psalm 89:27. David himself never actually gained dominion over all the kings of the earth, but Jesus, who is the fulfillment of God's promise to always have a man on the throne of David, is explicitly called "king of kings" and "lord of lords" in 1 Timothy 6:15 and Revelation 17:14. So it is perfectly appropriate to refer to Jesus as the firstborn in the same sense as David in Psalm 89:27. It fits. Moreover, the context of Colossians 1:15 appears to have this meaning in mind. It says,
He is the image of the invisible God, the firstborn of all creation. For by Him all things were created, both in the heavens and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things have been created through Him and for Him. He is before all things, and in Him all things hold together. He is also head of the body, the church; and He is the beginning, the firstborn from the dead, so that He Himself will come to have first place in everything. For it was the Father’s good pleasure for all the fullness to dwell in Him, and through Him to reconcile all things to Himself, having made peace through the blood of His cross; through Him, I say, whether things on earth or things in heaven.
Jesus is the firstborn of all creation, not because he's the first thing God made, but because all things were created by him, through him, and for him, and because in him all things hold together. Creation revolves around Jesus because it all exists by, through, and for him. That gives him preeminence over creation. Like Ephraim who was "before Manasseh"(Genesis 48:20), Jesus is "before all things" (Colossian 1:17). I'll come back to this point later.
Second reason
Jehovah's Witnesses believe that the phrase, firstborn of all creation, means that creation is the category and firstborn is a member of that category. So to say that Jesus is the firstborn of all creation means that Jesus is one of the things that is created. They are essentially making a grammatical claim.
There are instances in which firstborn is used as a member of a group. For example, Exodus 8:16 refers to the "firstborn of all the sons of Israel." Those firstborn are also sons of Israel. It's even used of livestock. Deuteronomy 15:19 refers to "the firstborn of your herd," and "the firstborn of your flock." In those cases, the firstborn is a member of the herd and of the flock.
But there is no grammatical rule that says a construction like Colossians 1:15 must mean that the firstborn is one of the things created. To be a grammatical rule, it would have to apply, not just to the noun, firstborn, but to anything that fits the pattern, noun1 pas noun2 where pas is the Greek word for all, every, etc. One time I searched the Septuigint and the New Testament (especially the letters of Paul) for instances of this pattern to see if noun1 was always part of noun2. I found lots of cases where noun1 was part of noun2, but I also found counter-examples. Here are a few.
John 17:2 "authority over all flesh"
2 Corinthians 1:3 "God of all comfort"
1 Peter 5:10 "God of all grace"
These counter-examples may not sit well with you because authority, comfort, and grace are all abstract nouns. We wouldn't expect authority to be a kind of flesh or God to be an item of comfort or grace in the same way that a firstborn might be a member of a herd. But there are better examples that resemble the pattern in Colossians 1:15 more closely. There's 1 Timothy 4:10 where Paul calls God "the savior of all men." That certainly doesn't mean that God is a man. Let's compare them.
1 Corinthians 1:15 "prototokos pases ktiseos"
1 Timothy 4:10 "soter panton anthropon"
Pas is spelled differently because one is feminine and the other is masculine, but they are both in the genitive case, which is why they are both translated, "of all." The "of" can mean either that one thing is a part of the other thing, or it can mean that one thing belongs to the other thing. In the case of God being the savior of all men, that doesn't mean God is part of mankind but that God is mankind's savior. In the same way, when Paul calls Jesus the firstborn of all creation, that doesn't mean Jesus is part of creation but that Jesus is creation's firstborn, i.e. creation's owner, heir, devisee, inheritor, ruler, head, authority, etc.
A third reason.
I keep editing this post to add stuff. Hopefully this will be the last edit. I wanted to respond to another reason Jehovah's Witnesses think the firstborn is part of creation. This argument is similar to the one before. It just doesn't make as strong of a claim. It's not a grammatical claim; it's an inductive argument from a pattern.
Here's the argument. Every other time you have "the firstborn of _____," it always turns out that the firstborn is part of the group. For example, when you see, "the firstborn of the flock," the firstborn is part of the flock. Therefore, when it says, "the firstborn of all creation," the firstborn is part of creation.
If that argument is sound, then so is this one: Every other time Jesus says, "Truly I say to you," the comma goes after "you." Therefore, when Jesus says, "Truly I say to you today you will be with me in paradise" (Luke 23:43), the comma should go after "you." But noooooooooooo, that's not where Jehovah's Witnesses put the comma.
I'm not trying to poo poo induction. Induction is how we discover grammatical rules after all. But there's a reason this pattern doesn't apply to Colossians 1:15. It's because of why the pattern exists. It exists because in the vast majority of cases, the firstborn refers literally to the first child, ox, sheep, or whatever born to its parents. With that being the case, we should expect the firstborn in most of these cases to be part of the group. But as I've already shown, that is not the only way firstborn is used in the Bible, and it certainly not the way it was used of Jesus. Jesus was not literally the result of procreation the way all the other firstborns in all these other cases were. Even if God did create Jesus, Jesus would still not be a firstborn in the same literal sense that human sons are literally the firstborn children of their parents since God doesn't have a wife that he procreates with and who gave birth to Jesus (unless you're a Mormon).
Back to the context
Lemme pick up where I left off above on Colossians 1:15-20. As I pointed out, the reason Jesus is the firstborn of all creation is because he created it all, and it all hangs together because of him. Since Jesus created everything, he himself can't be created. So the context indicates that the firstborn can't be part of creation. To get around this point, Jehovah's Witnesses insert the word other in various places. The New World Translation reads,
He is the image of the invisible God, the firstborn of all creation; because by means of him all [other] things were created in the heavens and upon the earth, the things visible and the things invisible, no matter whether they are thrones or lordships or governments or authorities. All [other] things have been created through him and for him. Also, he is before all [other] things and by means of him all [other] things were made to exist, and he is the head of the body, the congregation. He is the beginning, the firstborn from the dead, that he might become the one who is first in all things; because [God] saw good for all fullness to dwell in him, and through him to reconcile again to himself all [other] things by making peace through the blood [he shed] on the torture stake, no matter whether they are the things upon the earth or the things in the heavens.
That changes things. Whereas it would be complete nonsense to say, "Jesus is the first thing created because he created everything," it does make sense to say, "Jesus is the first thing created because all other things were created after him." However, if that is Paul's point, it's an awfully banal point. It would be like saying, "I'm the oldest child because all the other children are younger than me."
I don't think that's Paul's point, though. Paul's point, rather, is that Jesus is preeminent over creation because creation owes its existence to him. It was made for him, and he holds it all together. Paul is explicit in saying that Jesus is the "head of the body, the congregation." So it's Jesus' authority and preeminence that Paul is trying to explain.
Besides that, it appears that Paul is trying to be exhaustive in describing everything that owes its existence to Jesus. He says Jesus is responsible for things in "heavens and on earth, visible and invisible, whether thrones or dominions or rulers or authorities." That's pretty much everything in creation. Paul's contrast between heaven and earth, visible and invisible, are meant to be extremes, which indicates that Paul meant absolutely everything in creation. That excludes Jesus from being part of creation, and it means "other" doesn't belong in the passage.
Jehovah's Witnesses attempt to justify the insertion of other on the basis that it is warranted in passages like Luke 13:2 where Jesus said, "Do you suppose that these Galileans were greater sinners than all [other] Galileans because they suffered this fate?" Other is not in the Greek. It is added to the translation (NASB in this case) to complete the thought or make sense of the passage. Luke 13:2 warrants the insertion of other (though it's not necessary) because it explicitly tells us that it's talking about a small group of Galileans among all Galileans. Adding other to Luke 13:2 does not change the meaning of the passage; however, it does change the meaning of Colossians 1:16ff. Colossians is perfectly coherent without the insertion of "other," so it's not necessary to add it.
The use of other in Colossians 1:16ff both changes the meaning of the text and is also inconsistent with what the context suggests. So while other is called for in Luke 13:2, it does not belong in Colossians 1:16ff.
At this point, Jehovah's Witness will go to other passages they think indicate Jesus was created, then use that as a basis for their interpretation of Colossians 1:15. When they make this move, they are essentially abandoning their efforts to prove Jesus was created from Colossians 1:15. Instead of arguing from their interpretation of Colossians 1:15 to the creation of Jesus, they are instead arguing from their belief in the creation of Jesus to their interpretation of Colossians 1:15. If they still insist that Colossians 1:15 proves that Jesus was created, they are arguing in a circle. They are essentially saying, "We know Jesus was created because Colossians says so, and we know that's what Colossians says because Jesus was created." That's circular reasoning.
Having given up the use of Colossians 1:15 to prove that Jesus was created, the argument then falls to other passages, but that is beyond the scope of this post.