Read: John 1:1-3
Believing that Jesus Christ is, always was, and always will be fully God is a fundamental teaching of the Christian faith. It's very important. The first few verses of John is one of the most significant passages in the Bible that establishes this doctrine: the "Deity of Christ".
Verse one is very plan; however, Arian heretics pose an argument based on the absence of an indefinite article in the Greek. Because there's no "the" in Greek, the verse could be saying - they say - "...and the Word was a God."
This is really a smoke-screen, and not worth arguing with, because verse three leaves no doubt. Verse three implies there are two categories of existence: things that were made, and things that weren't made. Jesus wasn't made, because without him was not any thing made that was made. He couldn't have made himself - he wasn't made - he's in the "things that weren't made" category. There was never a time when Christ was not. There was never a time when the Father did not have The Son.
We worship God who became man.