Because he appointed a day during which he is going to judge the world in righteousness by a man whom he appointed, having provided assurance to all men because he raised him from the dead.
While he was still speaking, behold, a bright cloud overshadowed them. And lo, a voice out of the cloud, saying, This is my beloved Son in whom I am well pleased. Hear ye him.
He who believes in the Son of God has the testimony in himself. He who does not believe God has made him a liar, because he has not believed in the testimony that God has testified about his Son.
So that by two immutable events, in which it is impossible for God to lie, we may have strong encouragement, having fled for refuge to seize the hope being openly displayed.