I find in verses 15 and 16 a recap of the gospel: faith, repentance, baptism unto the remission of sins, endurance until the end. His testimony of these principles is confirmed to him according to the witness of the Holy Spirit.
Our fears are what bind us to destruction. Our faith is what causes us to live lives of extraordinary capacity and interest. There are sins which bind us down and commandments that liberate and free us. Commandments are not only those written in stone by Moses thousands of years ago, but also include the whisperings of the Spirit and "every word which proceedeth forth from the mouth of God." (Doctrine and Covenants 84:44)
It is also by the manifestation of the Spirit that Alma received a witness of the faith of those at Gideon (see vs. 17). This thought has been impressed upon me recently as to how this work is guided by the influence of the Holy Spirit. Obedience to that Spirit is requisite to be a true servant of the Lord. Alma places so much confidence in those impressions that he knows that his preaching to these people has not been in vain.
These verses also remind me of the instructions given in the Doctrine and Covenants concerning workings of the spirit: See Doctrine and Covenants 50.
Throughout these verses, Alma speaks in sure terms of knowledge and understanding using definitive terms like "I know" or "I perceive," followed by the explanation that the Holy Spirit is the source of his surety. Then in verse 20, Alma turns the tables with the following statement:
I perceive that it has been made known unto you, by the testimony of his word, that he cannot walk in crooked paths;Instead of stating that this is his testimony, Alma says that he perceives that this is the people's testimony. Testimony of what? A testimony of the Word of God that says that God cannot walk in crooked paths. So in other words, the people of Gideon had already obtained a testimony of the truth from their own application of the Word of God to their own lives and the conclusion that they had received -- the substance of their testimony -- is that when they do what God commands exactly as he has commanded them, then are they blessed. "Neither doth he vary from that which he hath said; neither hath he a shadow of turning from the right to the left, or from that which is right to that which is wrong;"
As a concluding thought, my perception of these verses has changed dramatically because of the time that I've taken to study them. What initially appeared to be as presumptuous statements, I now see as humble declarations of reality. It gives me pause to consider how many people falsely assume truth to be something other than that.