Actually it is not a separate event from the 2nd coming, the rapture is a smaller event that incompasses the larger event of the 2nd coming. The 2nd coming consist of the return of Christ from heaven to the earth, the resurrection of the just, the rapture of the living saints, the destruction of the wicked, and the setting up of the Millenniual Kingdom. So all 5 of these smaller event make up the larger event known to us as the 2nd coming. As for biblical proof for the rapture being a part of the posttrib 2nd coming and not a pretrib event.
6 points that prove post trib rapture (scripture included)
#1 Matt 24:29-31 and mark 13:24-27 say that the 2nd coming is after the tribulation and that there is a gathering of the elect at that time.
Matthew 24:29-31
29 Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of heaven shall be shaken:
30 And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory.
31 And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds from one end of heaven to the other.
Mark 13:24-27
24 But in those days, after this tribulation, the sun will be darkened, and the moon shall not give her light.
25 And the stars of heaven shall fall, and the powers that are in heaven shall be shaken.
26 And they shall see the Son of man coming in the clouds with great power and glory.
27 And then he shall send his angels, and shall gather together his elect from the four winds, from the uttermost part of the earth to the uttermost part of heaven.
#2 1st Thess 4:15-17 and 1st cor 15:20-23 say that the rapture is at the 2nd coming.
1st Thessalonians 4:15-17
15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep.
16 For the Lord himself shall descend with a shout, with the voice of an arch angel, and with the trump of God: and the dead will rise first:
17 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 we shall ever be with the Lord.
1st Corinthians 15:20-23
20 But now is Christ risen from the dead, and become the firstfruits of them that slept.
21 For since by man came death, by man came also the resurrection of the dead.
22 For as in Adam all die, in Christ shall all be made alive.
23 But every man in his own order: Christ the firstfruits; afterward they they are Christ at his coming.
#3 1st Cor 15:51-53 says that the rapture is at the last trump but matt 24:29-31 says that there is a trump sounded after the tribulation, the rapture can not be b4 this trump therefore can not be until after the trib is over.
1st Corinthians 15:51-53
51 Behold, I shew you a mystery; we shall not all sleep, but we shall all be changed,
52 In a moment, in a twinkling of an eye, at the last trump: for the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed.
53 For this corruptible must put on incorruption, and this mortal must put on immortality.
#4 Luke 17:26-30 says that the same day we are taken out is the same day Jesus returns to destroy the wicked (which he does at the end of the trib not the start) this is confirmed in 2nd thess 1:6-10.
Luke 17:26-30
26 And as the days of Noah were, so shall also the coming of the Son of man be.
27 They did eat, they drank, they married wives, and they were given into marriage, until Noah entered into the ark then the flood came and destroyed them all.
28 Likewise also as it was in the day of Lot; they did eat, they drank, they bought, they sold, they planted, they builded;
29 But THE SAME DAY that Lot went out of Sodom it rained fire and brimstone down from heaven and destroyed them all.
30 Even thus shall it be in the day the Son of man shall be revealed.
2nd Thessalonians 1:6-10
6 Seeing it is a righteous thing with God to recompense tribulation on them that trouble you;
7 And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels,
8 In flaming fire taking vengeance on them that no not God, and that obey not the gospel of our Lord Jesus Christ:
9 Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power;
10 When he shall come to be glorified in his saints, and to be admired in all them that believe ( because our testimony among you was believed ) in that day.
#5 John 6:39,40,44,and 54 all say Jesus said that he would raise us up at the last day (not 7yrs b4 ).
John 6:39, 40, 44 , and 54
39 And this is the Father's will which hath sent me, that of all which he hath given me I should loose nothing, but raise it up at the last day.
40 And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.
44 No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day.
54 Whosoever eateth my flesh, and drinketh my blood, hath eternal life, and I will raise him up at the last day.
#6 Rev 20 :4-6 says that the 1st resurrection is after the trib we know this because it includes tribulational martyrs as well as others that refuse to worship the beast or take his mark during the trib.There can be no resurrection of dead saints at a pretrib rapture b4 the 1st resurrection.
Revelation 20:4-6
4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshiped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands,; and they lived and reigned with Christ for a thousand years.
5 But the rest of the dead lived not again until the thousands years were finished. This is the first resurrection.
6 Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
Therefore the pretrib rapture as well as the mid trib is biblicaly impossible.