The apostle Peter likewise foretold of the apostasy:
But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction. And many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of. And through covetousness shall they with feigned words make merchandise of you: whose judgment now of a long time lingereth not, and their damnation slumbereth not.
(2 Peter 2:1-3)
John also spoke of these things:
Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time. They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us. But ye have an unction from the Holy One, and ye know all things.
(1 John 2:18-20)
John, whose writings are among the latest of the New Testament, speaks of the apostasy as having already begun. He speaks of it as "antichrist", which is a Greek term that means both "opposer of Christ" as well as "replacement of Christ". The antichrist spirit is the spirit that seeks to replace Christ and His spirit in the church - the very thing Paul spoke of in his second epistle to the Thessalonians.
I wrote unto the church: but Diotrephes, who loveth to have the preeminence among them, receiveth us not. Wherefore, if I come, I will remember his deeds which he doeth, prating against us with malicious words: and not content therewith, neither doth he himself receive the brethren, and forbiddeth them that would, and casteth them out of the church. Beloved, follow not that which is evil, but that which is good. He that doeth good is of God: but he that doeth evil hath not seen God.
(3 John 1:9-11)
Here, John describes a corrupted apostate elder in a local church, as one who "loves to have the pre-eminence". This is John's way of describing the man of sin who enthrones himself in the church in the place of God. Diotrephes is an example of the early burgeoning of the mystery of iniquity and the rise of the man of sin.
Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints. For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.
(Jude 1:3-4)
Jude likewise warned about the growing apostasy, and said it was necessary for him to remind the readers to earnestly contend for the original apostolic faith. Why? Because people had crept in (just like Paul warned would happen) and were perverting the true faith. The fact they were perverting the faith implies they were in a teaching position or some kind of leadership position in the church, again, just like Paul and Peter and John warned about.
And of course, we must ask where did the apostles get this idea from? where did they get this rather foreboding view of the future of the church, that it would include apostasy which would continue until the end? Well, it looks like they got it straight from Jesus Himself:
Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field: But while men slept, his enemy came and sowed tares among the wheat, and went his way. But when the blade was sprung up, and brought forth fruit, then appeared the tares also. So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares? He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up? But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them. Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.
(Matthew 13:24-30)