In a plan of surpassing beauty the Creator of the universe decreed the renewal of all things in Christ. In his design for restoring human nature to its original condition, he gave a promise that he would pour out on it the Holy Spirit along with his other gifts, for otherwise our nature could not enter once more into the peaceful and secure possession of those gifts. He therefore appointed a time for the Holy Spirit to come upon us: this was the time of Christ’s coming. He gave this promise when he said: In those days, that is, the days of the Savior, I will pour out a share of my Spirit on all mankind.
From a commentary on the Gospel of John by Saint Cyril of Alexandria, bishop