Jesus' mission on Earth was to spread the word of God and die on the cross as a penance for the sins of humans. According to John 17:18, God gave Jesus his mission. Jesus in turn gave his disciples, or followers, a mission on Earth to continue spreading God's word and the good news of Jesus' resurrection after he died on the cross.
The Bible passage in John 3:17 lets Christians know that Jesus is the savior that was sent by God to enable people to enter heaven and that his coming was not a statement of condemnation of the world. Jesus lived his entire life in a way that involved taking care of others. Even when he completed his mission on Earth, Jesus' death and resurrection allowed his mission on Earth to continue.
Jesus stated more than one time in the Bible that he knew that his mission on Earth would end on the cross. He told his disciples that he was sending them an advocate to help them continue when he was gone. His disciples went through the difficulty of Jesus' death and then continued on with his mission to spread God's word; they can still continue Jesus' mission on Earth.