3 Now behold ye know that this is true; and ye also know that an angel hath spoken unto you; wherefore can ye doubt? Let us go up; the Lord is able to deliver us, even as our fathers, and to destroy Laban, even as the Egyptians.
Nephi gives his brothers two really strong reasons to believe that God would deliver the plates and help them accomplish their mission. First, he reminds them that the Lord CAN deliver them, and he uses an historical reference to back that up. If the Lord can divide the Red Sea, surely He can deliver to us a set of plates. Second, Nephi reminds them that they just saw an angel of God. This would seem to be a strong indication that the Lord was behind the mission. In other words, not only CAN the Lord deliver us, He WILL deliver us, because this is His project. Given those two strong arguments, Nephi again asks why they are doubtful. For Nephi’s part, he was all about the mission from the beginning. After the angel left, there was no wasted time confirming and reiterating what he said about no beating their brothers. Nephi didn’t rub that in their faces, but instead he got right back on task: get the plates.