" And My servant whom I have chosen, That you may know and believe Me, And understand that I am He. Before Me there was no God formed, Nor shall there be after Me. (Isaiah 43:10, ibid)

God's answer is clear, that there was no God before Him nor shall there be after Him. So the teaching that Christ is the God who exist in the new testament time only not in the old testament time, is false and not biblical.

Now, did they know this truth that only the Father is the true God? Apostle Paul answer:

7 However, there is not in everyone that knowledge; ... (I Corinthians 8:7, ibid)

According to Apostle Paul, not everyone knows the truth about God. Who know this truth? In the verse 6, Apostle said this:

6 yet for us there is one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live. (I Corinthians 8:6, ibid)

Only the members of the Church of Christ know the truth.

Is Christ your God or is your God a Trinity, or do you believe in many Gods? Well, that is because you are not members of the Church of Christ. If you are only one of the members of Christ's Church, then you will uphold the teaching of Christ that only the Father is the true God and no God besides Him, nor there is another God like Him, nor there will be after Him. Only the Church of Christ know this truth.