How I see your reference here, "if" you have faith as in Abraham did, no question about it, He was going to do away with his Son.
He didn't necessarily no why etc, just went about it because he had faith that God knew best.
So then, because of his faith, by God's grace he was saved.
But if there was not the faith, it suggests, No saving.
Verses say, it is impossible to please God without faith.
Now this Grace, was always in the mind and plan of God (hence Abraham and Co rejoiced thinking about it, possible vision of it as well) and when the times were fulfilled it came via Jesus as in John 1:16:17.
Hence at John 1:15 is the context that John was referring to, "Jesus being preferred over John" as that was always God's first thought and plans. Hence Jesus being the beginning of the creation of God and slain before the foundation of the world = In mind and plan only, putting Jesus ahead of , or before John.
Expand
