en_tn/col/01/25.md

to fulfill the word of God

This means to bring about the purpose of God's gospel message, which is that it be preached and believed. "Word of God" here is a metonym for the message from God. Alternate translation: "to be obedient to what God has instructed" (See: rc://en/ta/man/jit/figs-metaphor and rc://en/ta/man/jit/figs-metonymy)