The ultimate source of Alma’s knowledge comes through the Holy Spirit. However, Alma makes certain to note that it was not a simple endowment of knowledge, but one he had to struggle for. He “fasted and prayed many days that I might know these things of myself.” He does not reference his particular conversion. It would appear that this need to know for himself grew from that experience, but he considered it separate from at least the visitation from the angel.
Equally as important to knowing that he spoke the truth is that he could testify that the fathers had spoken the truth. He had said, in verse 44, that “I am called to speak … and testify unto this people the things which have been spoken by our fathers concerning the things which are to come.” Alma declares that he not only so testifies, but that he has that testimony through the Spirit of God as well. He speaks of the spirit of prophecy because he is testifying of what verse 44 called “the things which are to come.” That is a phrase that points directly to the coming atoning Messiah.