“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.

The response of the 2 disciples to John’s identification of Jesus was to “follow” Him.  The word “follow” can be used in 2 ways:

1. to follow in a physical sense as down a path.
2. to follow as a disciple (learner). Both senses are in effect here.

This incident stands in contrast to that which is shown us in Matt 4:18-20, Mark 1:16-18.  The account of the calling of Andrew and Peter is later, after John the Baptist was put in prison (Mark 1:14).

Mark1:14-18  After John was put in prison, Jesus went into Galilee, proclaiming the good news of God.  “The time has come,” he said. “The kingdom of God is near. Repent and believe the good news!” As Jesus walked beside the Sea of Galilee, he saw Simon and his brother Andrew casting a net into the lake, for they were fishermen. “Come, follow me,” Jesus said, “and I will make you fishers of men.”  At once they left their nets and followed him.

What is the difference between the calling Andrew received to follow Jesus the first time and the second?  The call to follow Jesus and receive His salvation is for everyone who will hear Who He is and what He has done and act upon it; but God will, at His own discretion, place other calls upon an individual’s life (to become fishers of men, or apostles, prophets, teachers, etc. 1Cor 12:28-29, Eph 4:11).

1 Cor 12:28  And in the church God has appointed first of all apostles, second prophets, third teachers, then workers of miracles, also those having gifts of healing, those able to help others, those with gifts of administration, and those speaking in different kinds of tongues.

Eph 4:11  It was he who gave some to be apostles, some to be prophets, some to be evangelists, and some to be pastors and teachers,

What is the call God has placed on you?  Are you acting upon it as Peter and Andrew did when they left all to follow Jesus?