"Jesus is the 'firstborn of creation,' so he was created"

Colossians 1:15 calls Jesus "the firstborn of all creation." Firstborn means the first one born — so he is the first and highest creature, but still a creature.

In the Hebrew Scriptures "firstborn" had already become a title of status. God calls Israel his firstborn son though Israel was not his first nation, and he says of David, who was the youngest of eight, "I will make him my firstborn, the highest of the kings of the earth." The word marks pre-eminence and the right of inheritance. Paul uses it here to say the Son is heir and Lord over all creation — the firstborn over it, not within it.

You are reading a lot into one word. The plain sense is that he was born first.

Read the next two verses, because they are one sentence. "For in him all things were created, in heaven and on earth, visible and invisible… all things were created through him and for him. And he is before all things, and in him all things hold together." If the Son made every created thing without exception, he is not himself a created thing. And Paul says he is before all things and sustains them — language the Old Testament uses only of God.

The early church still debated this. Arius quoted Proverbs 8 and called the Son the first creature.

Arius did, and the church weighed it and rejected it — not by ignoring the texts but by reading them together. Athanasius spent much of his career on exactly this point: that "firstborn" and the Wisdom language of Proverbs 8 describe the Son's role and his eternal relation to the Father, not a beginning of his existence, and that a Son who created all things cannot be numbered among them. The Nicene phrase "begotten, not made" is the church's summary of that reading.