The church, as the body of Christ, began the day Jesus was raised from the dead and the spirits of all believers, were raised up together with Him. The body came up with the head, and that happened when God raised Him up and his body together with Him.
Ephesians 1:20-23 KJV
20. Which he wrought in Christ, when he raised him from the dead, and set him at his own right hand in the heavenly places,
21. Far above all principality, and power, and might, and dominion, and every name that is named, not only in this world, but also in that which is to come:
22. And hath put all things under his feet, and gave him to be the head over all things to the church,
23. Which is his body, the fulness of him that filleth all in all.
Ephesians 2:1-10 KJV
1. And you hath he quickened, who were dead in trespasses and sins;
2. Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience:
3. Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others.
4. But God, who is rich in mercy, for his great love wherewith he loved us,
5. Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved)
6. And hath raised us up together, and made us sit together in heavenly places in Christ Jesus:
7. That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus.
8. For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
9. Not of works, lest any man should boast.
10. For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.
Colossians 2:12-13 KJV
12. Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead.
13. And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;
Colossians 1:18 KJV
18. And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
1 Peter 1:3 KJV
3. Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead,
This would mean that the instant God raised up Jesus from the dead, all of the righteous dead in "hell" were raised up with Him as members of His body. That was the birth of the church, His body, in the paradise portion of hell. The living believers on earth (disciples etc.) were born again that instant, as the original church on the earth. Not on Pentecost, but on resurrection day.