There are various views regarding when the rapture will occur. The Apostle Paul specifically mentions the rapture in 1 Thessalonians 4:16-17. Paul continues by saying it will happen when Jesus comes as a thief (1 Thessalonians 5:1-2). After the sixth vial is poured out in Revelation 16:15, Jesus said that he is still coming as a thief thus indicating that the rapture will occur at Christ’s second coming.
But the strongest reason that I went from a pre-tribulation to a post-tribulation view is that Revelation 7:9 speaks of a great harvest of people that cannot be numbered who have come to follow Jesus Christ. The angel tells John that these came out of great tribulation (Revelation 7:13-14). When Jesus was here the first time He commanded His disciples to pray that the Lord of the harvest would send laborers into the harvest (Matthew 9:36-38).
God loves the world (John 3:16) and His heart is that none should perish (2Peter 3:9). If God raptured His people anytime prior to His second coming He would be removing large numbers of laborers from the greatest harvest of all time! Why would God do that and contradict the command He gave His disciples the first time He was here? God’s heart and focus is the great harvest.