Jesus replied, “You do not realise now what I am doing, but later you will understand.”
“realize” (NIV) = eido (Greek) = to perceive with the eyes, to know.
“understand” (NIV) = ginosko (Greek) = to learn to know, come to know, to know by experience. Maybe a reference to the illumination of the Holy Spirit (14:26, 16:13).
John 14:26 But the Counsellor, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you.
John 16:13 But when he, the Spirit of truth, comes, he will guide you into all truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come.
Peter would later understand. Perhaps after he had denied Christ when he never thought that he would, or perhaps after he learned on the shore of the Sea of Galilee what it really meant to feed the lambs of Christ’s flock, or perhaps not until he got older and could no longer dress himself or go where he wanted, but, whenever it was, Peter would most certainly come to understand exactly what Jesus was doing that night.