Does your church believe 1) God turned himself into a man and came to Earth in the form of Jesus? or 2) Jesus was with God in the beginning and was sent to Earth?
Of the two choices you give, most people in the churches of Christ would say they believe more that Jesus was with God in the beginning and was sent to earth. This is because there are specific passages to say that. I know few, if any, in the churches of Christ who believe that God gave up being God and became Jesus. We agree that Jesus was fully God, but not that he did not exist with God before coming to earth as a man. Some of the relevant passages are as follows.
"In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made. In him was life; and the life was the light of men. And the light shineth in darkness; and the darkness comprehended it not. … He was in the world, and the world was made by him, and the world knew him not. He came unto his own, and his own received him not. But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name: Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God. And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. (John 1:1-5, 10-14, speaking of Jesus)
"Your father Abraham rejoiced to see my day: and he saw it, and was glad. Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham? Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am." (John 8:56-58)
"And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ." (Ephesians 3:9)
"Who is the image of the invisible God, the firstborn of every creature: For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him: And he is before all things, and by him all things consist." (Colossians 1:15-17) (He is the firstborn of creation, and yet he is in nature God, being the "only begotten" son of God.)
"Let this mind be in you, which was also in Christ Jesus: Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross." (Philippians 2:5-8)