The Lamb was not slain before the foundation of the world. The plan that He would come to save man through his blood was before the foundation of the world.
In eternity, time has no bearing. In the fullness of time, what is known in the eternal is revealed.
If that Lamb was NOT slain from the foundation of the earth then how could Jesus forgive sins before His crucifixion?
Understanding that the Lamb was slain from the foundation of the world also explains why God would make Adam knowing he would sin: the price for Adam's sin was secured by the Lamb prior to Adam's creation.
This covenant was made between God the Father and God the Son, the promises of which vested with Abraham. This is what Hebrews 6:13-18 means:
"For when God made a promise to Abraham, because He could swear by no one greater, He swore by Himself, 14 saying, “Surely blessing I will bless you, and multiplying I will multiply you.” 15 And so, after he had patiently endured, he obtained the promise. 16 For men indeed swear by the greater, and an oath for confirmation is for them an end of all dispute. 17 Thus God, determining to show more abundantly to the heirs of promise the immutability of His counsel, confirmed it by an oath, 18 that by two immutable things, in which it is impossible for God to lie, we might have strong consolation, who have fled for refuge to lay hold of the hope set before us."
This connects to Genesis 1:26-28
"Then God said, “Let Us make man in Our image, according to Our likeness; let them have dominion over the fish of the sea, over the birds of the air, and over the cattle, over all the earth and over every creeping thing that creeps on the earth.” 27 So God created man in His own image; in the image of God He created him; male and female He created them. 28 Then God blessed them, and God said to them, “Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth.”
Adam sinned, so he did not measure up to the standard of the image and likeness of God (Father and Son). Adam lost his station. Yet, God still decreed that such a man would be so. God preordained a kinsman redeemer for Adam and his children before Adam was made. This redeemer would uphold the standard established by God and become the life-giving Spirit for all men. so that. whosoever believed in Him would be included in God's original design for man. This "Seed", the One to whom the promises were made, is Christ Jesus.
If I may, it went something like this:
Father and Son: We will make man in our own image and likeness.
Son: I will pay the price of their transgressions that have yet to be committed. Once revealed in time I will only do what I see You doing.
Father: Then, for all who receive you as their Redeemer I will likewise consider them my sons, as I consider you my Son.
I love you, Son, and I will give all things into your hands.
Son: Then what was decreed will be fulfilled:
"..let them have dominion over the fish of the sea, over the birds of the air, and over the cattle, over all the earth and over every creeping thing that creeps on the earth.”
This is the fellowship of the mystery which, from the beginning of the ages, was hidden in God and revealed in Christ:
"To me, who am less than the least of all the saints, this grace was given, that I should preach among the Gentiles the unsearchable riches of Christ, 9 and to make all see what is the fellowship of the mystery, which from the beginning of the ages has been hidden in God who created all things through Jesus Christ; 10 to the intent that now the manifold wisdom of God might be made known by the church to the principalities and powers in the heavenly places, 11 according to the eternal purpose which He accomplished in Christ Jesus our Lord, 12 in whom we have boldness and access with confidence through faith in Him."