Jesus said: “The Spirit of truth. He will come to you from the Father and will testify all about me. Moreover, you must also testify about me.” . With the pouring of the Holy Spirit in Acts 2 and the indwelling of him in the disciple at the moment of conversion (Ep. 1:13-14), the pupil is “empowered” to testify “with power” (Acts 1:8). A disciple is empowered to testify boldly about the redemption he has experienced through the atonement done at Calvary (Rom. 5:11).
In Mark 1:17-18, we see Jesus calling Simon and his brother Andrew, calling them to follow him. What is interesting here is how Jesus calls them. They were fishermen. They were doing their work when Jesus approached them and offered them a new job. Jesus promised them that they would now fish men instead of fish.
When the brothers heard Jesus, they acted according to what they heard Jesus speak of. Hel told them, “Come, follow me,” the NIV translates, “and I will send you out to fish for people” (Mr.1:17). They reacted with determination. The key word here is “immediately” (Mr.1:18). The Bible is clear in recording that it was “at once” that they quit what they were doing to follow Jesus. A disciple is characterized by his firmness of purpose. The brothers were determined to follow Jesus. So too modern-day pupils must be.
 Tyndale House Publishers, Holy Bible: New Living Translation (Carol Stream, IL: Tyndale House Publishers, 2013), Jn 15:26–27.