Jesus Christ is fully God (not half God or one-third God) and eternally God (He did not become God at some point in time). Without question, lacking this central pillar, Christianity fails and has nothing to offer anyone.
That Jesus Christ is God is disputed by other religions, by Mormons and Jehovah’s Witnesses, and by many skeptics.
The deity of Jesus is crystal clear if we allow the Bible to say what it says.
In the beginning there was the Word [referring to Christ]. The Word was with God, and the Word was God. John 1 v 1. This says that Christ existed before He was conceived by Mary, He has always been; He is eternal. The last phrase is also a straightforward statement of His deity.
Jesus said, ‘Before Abraham was even born, I am!’ (John 8 v 58). This was both a declaration of eternality and of deity, the Jews would have understood ‘I am’ as a reference to the name of their God. That is why they immediately sought to kill Him (v 59).
Jesus is omnipresent (demonstrated in His promise to be with all believers), omnipotent, omniscient, He created and sustains all things, gives life and forgives sins, things that God does.
Jesus said, ‘The Father and I are one’ (John 10 v 30).
Another unavoidable argument for Christ’s deity is that He accepted worship (such as John 9 vv 35-38 and John 20 vv 27-28). What is so amazing about these two examples is that these two loyal, strictly monotheistic, Jews did what they did. They worshiped Jesus because they had become completely convinced that this man is the one true God. And Jesus did not rebuke them. If thinking that He was God had revealed a terrible misunderstanding, Jesus could have said, as Paul and Barnabus did, and as angels repeatedly did. He went so far as to say ‘Anyone who does not honour the Son does not honour the Father who sent Him’ (John 5 v 23). In other words, if we don’t worship Him, we can’t worship God. Jesus accepted worship because He is God and is worthy of it.
One of the reasons that this doctrine is so crucial is that if Jesus is not fully God, there is no salvation to be found in His death.
‘All of God lives fully in Christ’ (Colossians 2:9).
‘The Son reflects the glory of God and shows exactly what God is like’ (Hebrews 1:3).
Condensed from Understanding Theology by Daryl Aaron.