The Lord Jesus Christ and the Apostles taught PSA, and wrote it for us. Many did not understand it since then, and many do not understand it today. The lamb slain from before the world was, is never popular among the religious world.
The Apostle warned of this in Acts20:
25 And now, behold, I know that ye all, among whom I have gone preaching the kingdom of God, shall see my face no more.
26 Wherefore I take you to record this day, that I am pure from the blood of all men.
27 For I have not shunned to declare unto you all the counsel of God.
28 Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.
29 For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.
30 Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them.
Paul spoke about the Lamb that was slain being resurrected and ascended unto the heavenly Throne ruling in His Kingdom
14 But this I confess unto thee, that after the way which they call heresy, so worship I the God of my fathers, believing all things which are written in the law and in the prophets:
15 And have hope toward God, which they themselves also allow, that there shall be a resurrection of the dead, both of the just and unjust.
16 And herein do I exercise myself, to have always a conscience void to offence toward God, and toward men.
17 Now after many years I came to bring alms to my nation, and offerings.
Yes Paul preached the cross;
17 For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.
18 For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.
23 But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness;
2 For I determined not to know any thing among you, save Jesus Christ, and him crucified.
7 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory:
8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
Yes, He preached the lamb slain, which was ordained before the world!