Many who do not accept that Jesus possessed full attributes of God point to the term “firstborn” when referring to Jesus as evidence that He was, quite literally, first created. Two verses used to support this idea are
Romans 8:29: “For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren.”
Colossians 1:15: “Who is the image of the invisible God, the firstborn of every creature.”
Referring to the second passage, Norman Geisler and Thomas Howe explain the term “firstborn” this way:
Paul clearly declared Christ to be God in this very letter by saying He “created all things” (1:16) and has the “fullness of the Godhead” (2:9). The reference to “firstborn” does not mean He is the first-born in creation, but the firstborn over creation (v. 15), since “He is before all things” (v. 17). “Firstborn” in this context does not mean the first one to be born, but the heir of all, the Creator and owner of all things. As Creator of “all things,” He could not have been a created thing. (When Critics Ask, p. 485)
Indeed, the term “firstborn” is a sign of preeminence. In Psalm 89:27, the term is used referencing David. In context, the passage says,
20 I have found David my servant; with my holy oil have I anointed him:
21 With whom my hand shall be established: mine arm also shall strengthen him.
22 The enemy shall not exact upon him; nor the son of wickedness afflict him.
23 And I will beat down his foes before his face, and plague them that hate him.
24 But my faithfulness and my mercy shall be with him: and in my name shall his horn be exalted.
25 I will set his hand also in the sea, and his right hand in the rivers.
26 He shall cry unto me, Thou art my father, my God, and the rock of my salvation.
27 Also I will make him my firstborn, higher than the kings of the earth.
David was “firstborn” not because he was the first created by his parents–in fact, David was the youngest of seven sons!–but because he was preeminent. Interestingly enough, Jesus was considered to be a “Son of David” because He was a direct descendant.
For more on this topic, see Does the Bible really say that Jesus is God?