32. Thus we see, if we believe we are to open our mouths and confess the grace God has shown us. This also is the greatest and best work of faith, namely, to inform and teach others in the Word; for as Paul says, Romans 10:10, “With the heart, man believeth unto righteousness; and with the mouth, confession is made unto salvation.” If one is ashamed of the Word and hides it, it is a sign of a lax faith.
The Sermons of Martin Luther, V,
Twenty-First Sermon after Trinity, Second Sermon
34. Thus we should teach one another to cling to his Word. For if we abide in his Word, we shall be sufficiently fortified against the devil; for we have a defiance of him in the Word, even though we ourselves are weak. But to the devil, who in an hour’s time could break in pieces all earthen vessels, all men would be as a feather, and he could blow them when and where he wished; but this feather shall become heavier for him than heaven and earth. For a Christian has Christ within himself; but Christ is heavier than heaven and earth. This must suffice concerning this Gospel.
From the same sermon.
