It is a fairly common opinion that the church began on the day of Pentecost (Acts ch. 2). Below are some of the reasons that I believe that the church was started before Pentecost. These may seem simplistic, but it doesn't take too much to satisfy a simple mind like mine.
1. The church is referred to before Pentecost, both by the word "church" (Matt. 16:18; 18:17) and by its figurative names - flock, bride, house, etc. (Luke 12:32, cf. I Pet. 5:2; John 3:29, cf. Eph. 5:22-31; Mark 13:33-36, cf. I Tim. 3:15).
2. The English word "church" is a translation of the Greek word "ekklesia", which means a called-out assembly. Jesus disciples were both called-out and assembling with Him before the day of Pentecost (e.g. Matt. 4:19; John 1:35ff).
3. The apostles were set in the church (I Cor. 11:28) and this occurred before Pentecost (Matt.10:1,2; Mark 3:13-19; Luke 6:13).
4. The ordinances were instituted and observed before Pentecost (John 4:1,2; Matthew 26:26-30).
5. John the Baptist prepared a people ready for the Lord Jesus Christ (Luke 1:17), not for the day of Pentecost.
6. The church was commissioned before Pentecost: first the limited commission of Matt. 10:1-4 and then the extended commission of Matt. 28:18-20.7. Jesus sang in the church before Pentecost (Heb. 2:12; Matt. 26:30).
8. The last days refer to the church age, and the last days were in existence during Jesus' ministry (Heb. 1:2).
9. There was church discipline before the day of Pentecost (Matt. 18:17).
10. The church had a business meeting before Pentecost (Acts 1:15-26).
11. The Lord added to the church on the day of Pentecost (Acts 2:41-27), so there had to exist a church for people to have been added to it.
12. There is no reason to suppose that the church could not exist with her visible Head present.
13. Nothing in scripture indicates the church was or had to be started on Pentecost.