There are various views regarding when the rapture will occur. The Apostle Paul specifically mentions the rapture:
For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive [and] remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord. (1 Thessalonians 4:16-17)
Paul continues by saying it will happen when Jesus comes as a thief.
But of the times and the seasons, brethren, ye have no need that I write unto you. For yourselves know perfectly that the day of the Lord so cometh as a thief in the night. (1 Thessalonians 5:1-2)
After the sixth vial is poured out, Jesus said that he is still coming as a thief thus indicating that the rapture will occur at Christ’s second coming.
Behold, I come as a thief. Blessed [is] he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame. (Revelation 16:15)
But the strongest reason that I went from a pre-tribulation to a post-tribulation view is that the Bible speaks of a great harvest of people that cannot be numbered who have come to follow Jesus Christ:
After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands; (Revelation 7:9)
The angel tells John that these came out of great tribulation:
And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they? And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb. (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:
But when he saw the multitudes, he was moved with compassion on them, because they fainted, and were scattered abroad, as sheep having no shepherd. Then saith he unto his disciples, The harvest truly [is] plenteous, but the labourers [are] few; Pray ye therefore the Lord of the harvest, that he will send forth labourers into his harvest. (Matthew 9:36-38)
God loves the world and His heart is that none should perish:
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. (John 3:16)
The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance. (2 Peter 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.