1 John 4:14 (ESV) … “And we have seen and testify that the Father has sent his Son to be the Savior of the world.”
John was writing to readers who probably never saw Jesus in the flesh. He was contending with false teachers who had claimed to have made heavenly journeys during which they saw God in heaven. John knew that this is impossible. God cannot be seen.
For John, he was working out how do you deny the claims of these teachers and still say, we have seen? How, if your readers have never seen Jesus, can they testify that God sent his Son to be our Savior?
First, John and his fellow apostles saw Jesus in the flesh, but the majority of those who saw Jesus did not join in the testimony. They cried for his crucifixion.
Second, such testimony is based on more than eyewitness. It comes through eyes of faith. Only after the resurrection did this testimony become real for the apostles. They testified to the church. Then the church accepted and repeated their testimony. We do the same.
Third, you do not have to see the earthly Jesus to testify about what God has done through him. You need only hear and believe in the testimony to him from Scripture and from faithful followers. Such testimony is both verbal testimony and God’s love exercised through our lives. The impact God has made in other Christians—this is what we have seen.
Based on the manifestation of Jesus in the lives of Christians, those who have witnessed it can testify that the Father sent Jesus to be the Savior of the world. That is, God sent Jesus to the cross to pay for our sins, so we do not have to suffer the wages sin pays, namely, death.
Testimony about Jesus tells more than what Jesus did—save from sin. It also tells who he is—the Son of God.