Question: In Col. 1:15, why does Paul call Jesus 'firstborn'? Did Paul not believe that Jesus was God?
AP: First things first, we have to point out that the vast majority of independent scholars do not believe that Colossians was written by Paul, rather by one of his disciples. If you're interested in finding out a little more on the subject, I have a book (Guía para entender el Nuevo Testamento) that you might find helpful. Now let's turn to your question.
Your question is a good one because it points out the inconsistencies of New Testament authors when it comes time to talk about the nature of Jesus as messiah. Theoretically, and from the point of view of theology today, that word ("firstborn") should not have been used without adding some additional information. But they saw no contradiction: Jesus is the firstborn in his role as a creature and in his role as God's mediator in the act of creation ... which is also self-contradictory if he is strictly God!
The problem is we are dealing with binitarianism, which is different from the ditheism. In current research, the Jewish monotheistic conception of divinity is called "binitarianism" and is clearly distinguishable from any case of "ditheism." Binitarianism is the belief that puts forth the existence of one God, a belief that wants to preserve this being's transcendence in in relationship to the world; therefore, it relies on an agent that is "on the side" and is subordinate to act "outside" (i.e., the Wisdom, the Word/Logos). Such an agent is a divine figure and is related to the first; but the first is fully God, while the second is only part of that divinity.