Why Did God not prevent Adam’s fall, since He had foreseen it?

Had He prevented it, He would have interfered and would have abolished man’s freedom, which He bestowed him as a gift. Had He removed man’s freedom, then man’s conduct as well as his salvation would have been compulsory. Man would have lost his personality and would have been a creature without will. God preferred to change His designs on man, rather than take away the most significant element of his personality, his freedom.

God has added a second element which is beneficial to man: His justice against demonic malevolence and hate. The devil believed that by misleading man, he would have prevented God’s designs and would have shattered man’s likeness to God. Thus he believed he would have managed to both take revenge on God and deprive man of his value. Thus, God did not prevent the devil from implementing his evil plan in order to completely crash him when He took upon Himself human nature, by His future incarnation. In this way, man who was the victim of demonic wickedness is to be able to rise “far above all rule and authority and power and dominion, and above every name that is named” (Ephesians 1, 21) not only in this age but in the years to come. Therefore, had God prevented man’s fall, He would have deprived him of the glory which he has inherited by his substantial union with God himself, through His incarnation.

Excerpts from the book ″Discourse on Mount Athos″ by Elder Joseph of Vatopedi.
Translated by Olga Konari Kokkinou from the Greek edition: Γέροντος Ιωσήφ Βατοπαιδινού, Συζητήσεις στον Άθωνα, Ψυχοφελή Βατοπαιδινά 13, Ιερά Μεγίστη Μονή Βατοπαιδίου, Έκδοσις Α’ 2003