I said
well if the Father is greater than Jesus. That means the Father is the only God.
If Jesus is God, why did Jesus say the Father was greater than He (John 14:28)?
He said this because His position was different from that of God the Father--not His nature. Heb. 2:9 says that Jesus was made for a little while lower than the angels, and Gal. 4:4 says that He was under the Law. Therefore, as a man, He was in a lesser position than the Father but not different in nature. This would also explain why He grew in wisdom and stature (Luke 2:52).
By comparison, a husband is the head of the family, and the wife is not. Though their positions are different, he has greater authority. Their natures are the same. This is how it works with Jesus. His nature is the same as the Father, but He was sent by the Father (John 6:44) and was in a lesser position due to His incarnation and being under the Law.