When donning the armor of God, one realizes that there are different roles for each piece of the armor. The shield represents faith, the helmet represents salvation, and the sword represents the Spirit and the word of God (Eph 6:16-17). Interestingly, the sword represents both the Spirit and the word of God. More descriptively, one could say that it represents the word of God as taught by the Spirit, or in other words, pure testimony.
Of all the many pieces of armor, this is the only offensive weapon available to the soldier in the Lord’s army. All the other pieces of armor are defensive. We should not find it surprising that Alma is going on the offensive with the only weapon available to him—the Spirit and the word of God of his pure testimony against them. All great missionaries have used this most effective and useful weapon in the fight against wickedness.
Orson F. Whitney
“…so long as we have boys (acting as missionaries), striplings, who can speak by the power of God, who can testify by the Holy Spirit, we need not despair. Mormonism has in its hand the mightiest weapons that man can wield, divine authority and the power of pure testimony that cuts like a keen two-edged sword. Argument has its mission, and God can inspire an argument just as readily as He can a testimony; but there is something peculiar about the power of testimony. It is a pioneer. Argument may come afterwards and fill up the gaps, build the bridges and the cities; but testimony goes before into the wilderness blazing a trail, and marking out the way. Joseph Smith said, ’I saw and I heard,’ and who can gainsay it, especially when the words are accompanied by the mighty power, the convincing power of the Holy Ghost? No argument can stand against it, and there is enough force and virtue in such a testimony to bring the whole proud world under condemnation if they reject it.” (Conference Report, Oct. 1912, p. 47)
Bruce R. McConkie
"…it remains our responsibility, I think, to teach the doctrines of the kingdom, to expound the principles of salvation to the world. Our time is too important to teach ethical platitudes. We are expected to give all men to whom we have opportunity to give it, the message of salvation, the glad tidings of the restoration, the fact that God has spoken in this day, and the assurance that there is peace and joy and happiness by living the gospel here and now, and an eternal reward in the world to come.
“Then after we have taught people the principles of the gospel, after we have let our light shine before them, it remains for us to seal that witness with pure testimony, as moved upon by the Holy Ghost, that we as individuals know that these things are true.” (Conference Report, Oct. 1948, p. 47)
“Faith comes by hearing the word of God, through the testimony of the servants of God; that testimony is always attended by the spirit of prophecy and revelation” (Teachings, p. 148)"