What does it mean for Christ to have been slain before the creation of the world?

Just like St. Augustine f.e., I believe that God lives outside of time. Unlike him, I have come to know that time is but another dimension in our universe, the fourth dimension of space-time. So time, as shown in relativity theory by Einstein, can f.e. be bent just like any other of the four dimensions.

But if God lives outside of time, he can observe time just as we can observe space. When we look at things, we can pretty much decide upon the perspective we take and view the whole stretch of, let's say, the length of what we are looking at, if the object is small enough to not escape our view port.

Now, the Bible tells us that God holds the universe between his thumb and his forefinger. That would by no means escape his viewport.

Let us make an example. I can touch the middle of the desk in front of me, and then touch it's left end, before touching its right. If time is only a dimension, and I am not a slave of time, that means subjected to it as we are (we experience time as a half dimension, since we can only travel through time forward, and are not free to select the speed in which we travel), God can touch time in whatever order he wants.

In that scenario, Christ can easily be slain before the world even exists, touching a later point in time (after creating it in the first place, since, as a dimension of this universe, it did not exist before creation). One could view it just like the creation in Gen 2: God made man, and then put him into the garden. He made man before anything grew (maybe even before plants even were created), but put him into creation in the afternoon of the sixth day (i.e. a much later time).

Thinking this through, the experience of the cross becomes something outside of time. The moment I accept the work of Christ at the cross, the cross happens for me. This event outside of time becomes real and manifests in time at a set time.

God knows the end from the beginning. (Isa 46:10) God planned it all before time was. (Eph 1:4)

This is how God works. We pray, and he puts the answer into our future. We prophecy, as he reveals to us what he put into the future already. He said: it is finished, as he had accomplished all, including future things.

The Bible declares that Jesus is the Express image of God's person, meaning that if you had to put a face to God it would be the face of Jesus. God is invisible in Spirit but came and made Himself visible through the Person of Jesus Christ. He is not just the Son of God but He is the express image of God Himself. Therefore, when Jesus was slain before the foundations of the world God already had His plan for salvation in mind. The Perfect lamb Jesus Christ would come and die for the sins of His people. Now if God had not put on flesh to redeem us we would never have been saved because the Bible declares that without the shedding of blood there can be no remission of sins. But not just any blood because this blood had to be pure and sin free, so the only man that was without sin was God when He put on flesh because the bible says that we were all conceived in sin. So how can it be that Jesus was the only one who had no sin? Because He is God manifested in flesh 1 Tim 3:16 and John 1:1 & vs 14

God being who he is. I believe he knew man would fail in the garden. Christ is often referred to as God in the old testament. Remember John said in the beginning was the word the word was with God and the word was God, it goes on to say the world was made by him yet he was in the world and the world didn't know him. It said everything was made by him. Now some say their is a contradiction. When the bible says no man has seen God. Yet who was the one all these people in the old testament seeing. Well it was Christ in his glory. Think of us as a person we have a body of flesh. That is Christ we have a soul that is our core. That's is father God. We also have a spirit which is a vehicle in which we move after this flesh dies. Well that's God Holy Spirit. Yet Christ is the form God chose to interact with man. So he was chosen by the Father to die for man before the foundation of the world was ever made. Thus slain before the foundation of the world. Because he was already chosen to to be the slain sacrifice for man. Hope this helps.

The reason it is written that Jesus was slain before the foundation of the world is to draw our attention to his deity. Jesus is not restricted to time and therefore everything has already been completed throughout eternity. Jesus reveals himself in the flesh for our witness: "Who verily was foreordained before the foundation of the world, but was manifest in these last times for you,"1Pet 1:20. Because Jesus was crucified before the foundation of the word, this gave way for those in the Old testament to "drink of the same spiritual Rock," as we drink from today-that Rock is Christ.(1Cor 10:4). How else could the gospel have been preached to Abraham(Gal 3:6-8). Jesus said, "before Abrahams was, I am,"(John 8:56).

Notice the reading of the King James Version:

Revelation 13:8 (KJV) And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world.

Modern Translations read differently:

Revelation 13:8 (ESV) and all who dwell on earth will worship it, everyone whose name has not been written before the foundation of the world in the book of life of the Lamb who was slain.

Revelation 13:8 (NLT) And all the people who belong to this world worshiped the beast. They are the ones whose names were not written in the Book of Life before the world was made--the Book that belongs to the Lamb who was slaughtered.

Revelation 13:8 (NASB) All who dwell on the earth will worship him, everyone whose name has not been written from the foundation of the world in the book of life of the Lamb who has been slain.

It seems that modern translations show that the Lamb was not slain before the foundation of the world.

There are other references in Scripture that present this same phenomenon of the apparent eclipse of time. For instance, in Revelation 13:8, Jesus is referred to as "the Lamb that was slain from the creation of the world." Now the cross occurred at a precise moment of history. We know when the Lamb of God was slain. But the Bible says it occurred before the foundation of the world. How can an historical event, which occurred at a certain spot on earth, in the biblical reckoning be said to have occurred before the earth was even made? The passage does not say that the Lamb was foreordained to be slain before the foundation of the world, but it says He was actually slain then. Surely it means that the cross was an eternal event, taking place both in time and eternity. In time, it is long past; in eternity, it forever occurs.

The same is true of the resurrection and even the second coming of Christ. When any Christian dies, he passes from the realm of time and space into timelessness, into the NOW of God, when the full effect of these timeless events is experienced by him to whatever degree his spiritual state requires. But the Lord's return is an event yet to take place in historical time when the Church is complete and the end of the age has come. Perhaps this is the meaning of the Lord's words: "I tell you the truth, a time is coming and has now come when the dead will hear the voice of the Son of God and those who hear will live" (John 5:25).

A problem passage for some, in this respect, has been Revelation 6:9-11 where John sees the souls of those who had been slain for the Word of God under the altar in heaven. They are crying out to God, "How long, Sovereign Lord, holy and true, until you judge the inhabitants of the earth and avenge our blood?" In response they are told to be patient a little longer until the full count of martyrs is complete. This seems to indicate a sense of time in heaven and a need to wait for something in the future. How do we explain this in the light of what we have just seen regarding time and eternity?

The explanation, of course, is that John, who sees all this, is still a man living in time and space on earth. It is necessary, therefore, that what he sees in heaven be communicated to him in the symbols and language of earth. This is a common phenomenon in the Book of Revelation. In the first chapter John sees Jesus in heaven. Does he really have long white hair and feet like burnished bronze and does a sharp sword come out of his mouth? No, clearly these are symbols which convey to John the power, wisdom, and glory of the Lord Jesus in his glorified, risen estate. The truth conveyed by the vision of the souls under the altar is evidently their identification with and concern for their brethren who are still on earth. They express themselves in terms of time and space in order that John (and we) may understand.

Mark Edwards has an excellent explanation of Romans 13:8 on the Biblical Hermeneutics Stack Exchange. In his excellent answer, he notes:

...the exact same idiom, right down to the noun morphology, is found in five places in the new testament outside of the Revelation. Because the wording is used identically in all seven instances, this greatly suggests the idiom had taken on a systematic meaning in Greek-speaking Jewish-Christian thought in the first century.

He concludes by saying:

If we follow the way the idiom in the third clause is used in other first-century Christian literature, as well as a parallel in the Septuagint, the phrase connotes the time after the world was made, not before.

And in fact, his Septuagintal comparison was the most elucidating in this case for me:

Psalm 78.2: I will utter riddles from ancient times.

LXX Psalm 77.2: I will utter riddles from the beginning [απ αρχης].

Matthew 13.35: I will utter [things] hidden απο καταβολης κοσμου.

What I conclude from this is that just as the riddles come from ancient times, so does Jesus sacrifice. Just as the riddles are uttered in the present, Jesus was not actually sacrificed before or at the creation of the world, but the origin of his sacrifice comes from the creation of the world. In other words, the sacrifice comes from (is made because of) the fall of man; Adam and Eve's sin.