First, we should mention that those reverend Apostles had been called for their mission by Jesus Christ himself. He is the One who said to them:" You did not choose Me, but I chose you and appointed you that you should go and bear fruit, and that your fruit should remain, that whatever you ask the Father in My name He may give you" (Jn15:16)
And Jesus Christ also said to them after the Resurrection" As the Father has sent Me, I also send you"(Jn20:21).
How gracious it is that God's servants are called to the service by God Himself. St. Paul the Apostle said in his Epistle to the Hebrews "And no man takes this honor to himself, but he who is called by God, just as Aaron was"(Heb5:4).