Commenting on 1 Peter 1:20, Calvin addresses this question: Was Christ our salvation an afterthought of God?
It may be asked, As Adam did not fall before the creation of the world, how was it that Christ had been appointed the Redeemer? for a remedy is posterior to the disease. My reply is, that this is to be referred to God's foreknowledge; for doubtless God, before he created man, foresaw that he would not stand long with integrity. Hence he ordained, according to his wonderful wisdom and goodness, that Christ should be Redeemer, to deliver the lost race of man from ruin. For herein shines forth more fully the unspeakable goodness of God, that he anticipated our disease by the remedy of his grace, and provided a restoration to life before the first man had fallen into death. (Commentaries on the First Epistle of Peter, 52, emphasis added.)
In his awesome sovereignty, God only ordained one plan of salvation through eternity; namely, Jesus Christ and him crucified. Amen.