The next step is that you must obtain spiritual knowledge. In this passage Alma testified, "Do you not suppose that I know of these things myself? And how do you suppose that I know of their surety?" Interesting that Alma uses this word surety here. "To know of a surety." When he first experienced his conversion, he knew! He said, "I have been born of God. I know." And in Alma 36 later said, "For ye ought to know as I do know" (Alma 36:30), but that was not all that Alma ever learned.
The Holy Ghost reinforced and taught Alma more, as he fasted and prayed many days, kept the commandments, served, and lived righteously. There came a greater level of knowledge. He gained an assurance, a surety, and he knew at more than just the experiential level of having been called to repentance.
Ed J. Pinegar and John W. Welch, Experiencing a Mighty Change of Heart: Alma’s Guide to a Deep, Lasting Conversion (American Fork, UT: Covenant Communications, 2020), chapter 8 "Receiving Personal Revelation."