A personal testimony does not replace a biblical proclamation about Jesus, but it is an important complement. And it requires that we have a close relationship with the Lord. If we are not excited about God’s Word, if we are not warmed by close fellowship with God, and if we are not humbled by Christ’s suffering on the cross for our sins, we will not be very effective witnesses. Yet it is essential that we be able to give such a witness. MacArthur is right when he says:
Most people do not come to Christ as an immediate response to a sermon they hear in a crowded setting. They come to Christ because of the influence of an individual…. In the overwhelming majority of [new believers' testimonies], they tell us they came to Christ primarily because of the testimony of a coworker, a neighbor, a relative, or a friend…. There’s no question that the most effective means for bringing people to Christ is one at a time, on an individual basis.
Between [Peter and Andrew] we see the two main kinds of witnesses God provides in the church: the public preaching of the Word and the personal testimony of individual Christians. Every church needs a Peter who will preach the gospel publicly, and God greatly uses faithful preaching. Peter’s sermon at Pentecost, when three thousand people believed on Christ, is one such example. But as important as preaching is, it is at least as necessary that a church have a legion of Andrews: those who bring people to Jesus one by one through their heartfelt testimonies.
Richard D. Phillips, Jesus the Evangelist (Kindle Edition, location 573)