Why could the Rapture be considered not biblical?
The only aspect of the “Rapture” that is not biblical, is the word itself. There are two events that the are referred to with respect to coming of Christ. The First Advent is the Rapture, which from a biblical perspective is chronicled in 1 Thessalonians 4, 1 Corinthians 15. and Matthew 24 & 25
This is where Christ returns and all Christians are taken out of the World at that time. The reference Jesus gave are found Matthew 24:40 and Matthew 25:1–13
35 Heaven and earth shall pass away, but my words shall not pass away.
36 But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only.
37 But as the days of Noah were, so shall also the coming of the Son of man be.
38 For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark,
39 And knew not until the flood came, and took them all away; so shall also the coming of the Son of man be.
40 Then shall two be in the field; the one shall be taken, and the other left.
41 Two women shall be grinding at the mill; the one shall be taken, and the other left.
42 Watch therefore: for ye know not what hour your Lord doth come.
1 Then shall the kingdom of heaven be likened unto ten virgins, which took their lamps, and went forth to meet the bridegroom.
2 And five of them were wise, and five were foolish.
3 They that were foolish took their lamps, and took no oil with them:
4 But the wise took oil in their vessels with their lamps.
5 While the bridegroom tarried, they all slumbered and slept.
6 And at midnight there was a cry made, Behold, the bridegroom cometh; go ye out to meet him.
7 Then all those virgins arose, and trimmed their lamps.
8 And the foolish said unto the wise, Give us of your oil; for our lamps are gone out.
9 But the wise answered, saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves.
10 And while they went to buy, the bridegroom came; and they that were ready went in with him to the marriage: and the door was shut.
11 Afterward came also the other virgins, saying, Lord, Lord, open to us.
12 But he answered and said, Verily I say unto you, I know you not.
13 Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh.
1 Thessalonians 4:15-18
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 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:
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 shall we ever be with the Lord.
18 Wherefore comfort one another with these words.
1 Corinthians 15:50-55
50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.
51 Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,
52 In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
53 For this corruptible must put on incorruption, and this mortal must put on immortality.
54 So when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory.
55 O death, where is thy sting? O grave, where is thy victory?