Five of the disciples had already encountered Christ. At Christ’s baptism John the Baptist had directed two to them to go to him, Andrew and John.
The call to serve the Lord is the most wonderful call possible. It is a call to leave behind the mundane existence we had before, when we were necessarily involved with things that are passing away, and to begin a new life which is dedicated far more to the service of Christ. Although all are to do what they do as unto the Lord, the one who serves him full time can devote himself to what is directly and intimately connected to the advance of the kingdom of heaven.