Am I showing disrespect toward you or your belief system? If I am not please do not disrespect me, by cussing.
Jesus was God in the flesh. Colossians 1:15 (NLT) "Christ is the visible image of the invisible God. He existed before anything was created and is supreme over all creation..."
Also John saw Christ in heaven after Jesus Resurrection thus we see in the last book of the Bible called Revelations. "...his servant John, who bore witness to the word of God and to the testimony of Jesus Christ, even to all that he saw....."
So since John saw the Eternal God in all His glory he(John) has validation towards his claim.
Now about nobody being around before eternity,that would make since since we mere mortals are not eternal. We should expect an Eternal Being who claims to be the only Eternal Being not to have any other beings around Him before there was time.
The Bible clearly teaches "the just shall live by faith" (Habakkuk 2:4, Romans 1:17, Galatians 3:1, Hebrews 10:38) so I take by faith that God who can not lie is eternal. Just like by faith you believe what you believe about the eternality of God, but will you ever be or do have any real hope that you will be justified by your faith in the mathematic improbability that nothing created something?