As you study the establishment of the church, you will often first come to Matthew 16:18 where Jesus says, “I will build my church.” We know that Jesus is the one that does and will build his church. He is the one who paid the price for it and the owner of it.
As you read through the Gospel of John and see Jesus training his disciples and then read in Acts 1:8 that Jesus tells them they will be his “witnesses” to the world, you can easily put together that the disciples time with Jesus was not only to believe on him as the Son of God but so that they could be a witness unto all people that Jesus is the Savior of the world
“This plan, …, was that He would build his church. For the disciples, the implications of this was that they had to go into all the world and to disciples all the nations.” 
Therefore, we can see two main aspects of this “witness.” We see the proclamation of the witness and the people who are the witnesses. Through these two aspects of this WITNESS Jesus will and does build his church. The purpose of WITNESS is for Jesus to build His church. He uses the message (the good news of the witness) and the messenger (those who received and testify of the good news) to cause others to believe and become a part of the church.
“We may safely conclude that the purpose of witnessing is to convince the nations about the truth of the gospel and to bring them to believe in Jesus”