Why was the lamb slain at the foundation of the world? So that light could become darkness, good could become evil, the sinless could become sin, and we could have sentient life with free will.
The Lamb of God
Foundation: καταβολῆς – katabolé
The "foundation-plan" typically relates to Christ's incarnation, i.e. coming to earth in the flesh to be our Redeemer. This divine plan was set and sealed (guaranteed) before creation.
Conception: καταβολῆς – katabole
The foundation of the world was an act of conception. Wisdom (Hydrogen, H+) fused with the Holy Spirit (alpha Oxygen, αO2-) to form the Hydroxyl ion (OH-). This existed with Wisdom (H+) to be the deep (H2O).