"This testimony" is God's testimony about His Son. Judging by the context, this refers to the episode of Jesus' baptism (Matt. 3:16-17), when God's voice from heaven called Jesus the beloved Son. Let us notice the chain of witnesses that brings this news to us: the Father bears witness to the Son, John the Baptist bears witness to this to his disciples, and the disciples bear witness to this testimony to us through the Gospels. Testimony is the main way the good news is transmitted, because in it, for all its truth, there is no coercion of the listener.
What is this testimony about? "In Him is My good pleasure," that is, the good will of the Father is revealed in the Son. What, then, is this good will of God? Look at several passages: Ezek. 33:11, Matt. 18:14, John 3:16 - and you will see that God wants to deliver us from death and give eternal life. Therefore, in Jesus Christ the Father's will for eternal life for human beings is revealed: the Son of Man, passing through death into resurrection, ascended to the Father and abides forever at the Father's right hand. This means that our life can become eternal if we are united with Christ and allow Him to fill us with His presence and His life.
