The Authority of Jesus Is Revealed in The Calling of the First Apostles (1:16-20).
The last thing that anyone would have thought of when they heard Jesus' message about the Kingly Rule of God would be that He would then call on a group of professional fishermen to have a part in the transmission of that message. It is as though Mark is emphasising the lowliness of the beginnings of God' Kingly Rule. First the Coming One Himself is from lowly Nazareth, now the heralds are lowly fishermen.
But Simon (Peter) and Andrew had both already expressed their interest in Jesus, the latter at least having been a disciple of John the Baptiser (John 1:40-42). And John (the Apostle) was also probably with them in those early days as the unnamed of the two disciples of John (John 1:35). While John the Baptiser was ministering Jesus would not officially call them (as He did call Philip - John 1:43), for they were John's disciples, and thus after the imprisonment of John they had returned to their homes and their livelihoods. Now Jesus sought them out and officially called them to ‘follow Him', that is, to commit their lives to hearing and responding to His teaching, so that from that moment on they were to have complete trust in Him and were to be declarers of God's Kingly Rule.
Mark sees this as a further step in the revelation of the Kingly Rule of God. The anointed representative of the King, indeed the King Himself (John 1:49), is seen as having the right to call men to leave everything and follow Him, to assist in the task that is now His. He deliberately makes his account stark and demanding (compare Luke 5:1-11), revealing that the One sealed by God, has the right immediately to demand what He will. It is the demand of a king.
Discipleship was a common feature in Palestine. The Rabbis had their disciples who came and learned from them and followed them. But they did so by choice, and they were not specifically called on to leave all. Jesus' call to follow Him was, however, all embracing and permanent (Mark 9:34-37 compare Matthew 19:29). It was the call of One with sovereign rights. He spoke as One Who brooked no refusal and all the emphasis is on this. They must follow Him all the way, for there was a work for them to do in the future.
Analysis of 1:16-20.
a And passing along by the sea of Galilee, he saw Simon and Andrew the brother of Simon casting a net in the sea, for they were fishermen. And Jesus said to them, “Come you after me, and I will make you to become fishers of men” (Mark 1:16-17)
b And immediately they left the nets, and followed him (Mark 1:18).
a And going on a little further, he saw James the son of Zebedee, and John his brother, who also were in the boat mending the nets. And immediately he called them, and they left their father Zebedee in the boat with the hired servants, and went after him (Mark 1:19-20).
Note that in ‘a' we have the call of Simon and Andrew, and in the parallel the call of James and John. Centrally in ‘b' we have the result of the call of Simon and Andrew (with James and John the result of the call is not a separate statement).