Alma 5:13

And behold, he preached the word unto your fathers, and a mighty change was also wrought in their hearts, and they humbled themselves and put their trust in the true and living God. And behold, they were faithful until the end; therefore they were saved.

After we receive a witness from the Holy Ghost, we must stay humble, trust in God, and stay faithful to keep it. President Ezra Taft Benson explained: “Repentance means more than simply a reformation of behavior. … True repentance is based on, and flows from, faith in the Lord Jesus Christ. There is no other way. True repentance involves a change of heart and not just a change of behavior” (The Teachings of Ezra Taft Benson 1988, 71).

Mosiah 5:2

And they all cried with one voice, saying: Yea, we believe all the words which thou hast spoken unto us; and also, we know of their surety and truth, because of the Spirit of the Lord Omnipotent, which has wrought a mighty change in us, or in our hearts, that we have no more disposition to do evil, but to do good continually.

One form of a testimony is a mighty (powerful) change of heart that makes evil unattractive and creates in us a desire to do only good. The natural man, the enemy to God, loses his grip on us, and the Spirit of God gets a firmer hold upon our hearts and our desires. Remember that king Benjamin's people were already believers. So this mighty change can happen to us throughout our lives in the Church. It can happen many times and any time that we try to get closer to the Lord.