YUSUFALI: When Jesus found Unbelief on their part He said: "Who will be My helpers to (the work of) Allah?" Said the disciples: "We are Allah's helpers: We believe in Allah, and do thou bear witness that we are Muslims.
Sound like a vision from God turned his life around and he became a messenger.
Crucifixion if I understand it correctly was a public execution in a highly visible place. The fact the apostles fled does not mean that others did not witness the crucifixion, nor that the apostles did not watch from a distance. Acts says it was widly known.
Lk 24:13 Now behold, two of them were traveling that same day to a village called Emmaus, which was seven miles from Jerusalem. 14 And they talked together of all these things which had happened. 15 So it was, while they conversed and reasoned, that Jesus Himself drew near and went with them.
16 But their eyes were restrained, so that they did not know Him. 17 And He said to them, "What kind of conversation is this that you have with one another as you walk and are sad?" 18 Then the one whose name was Cleopas answered and said to Him, "Are You the only stranger in Jerusalem, and have You not known the things which happened there in these days?" 19 And He said to them, "What things?" So they said to Him, "The things concerning Jesus of Nazareth, who was a Prophet mighty in deed and word before God and all the people, 20 "and how the chief priests and our rulers delivered Him to be condemned to death, and crucified Him. 21 "But we were hoping that it was He who was going to redeem Israel. Indeed, besides all this, today is the third day since these things happened. 22 "Yes, and certain women of our company, who arrived at the tomb early, astonished us. 23 "When they did not find His body, they came saying that they had also seen a vision of angels who said He was alive. 24 "And certain of those who were with us went to the tomb and found it just as the women had said; but Him they did not see."
25 Then He said to them, "O foolish ones, and slow of heart to believe in all that the prophets have spoken! 26 "Ought not the Christ to have suffered these things and to enter into His glory?" 27 And beginning at Moses and all the Prophets, He expounded to them in all the Scriptures the things concerning Himself.