Jamieson, Fausset and Brown on Romans 1:16

16. For I am not ashamed of the gospel--(The words, "of Christ," which follow here, are not found in the oldest and best manuscripts). This language implies that it required some courage to bring to "the mistress of the world" what "to the Jews was a stumbling-block and to the Greeks foolishness" (1Co 1:23). But its inherent glory, as God's life-giving message to a dying world, so filled his soul, that, like his blessed Master, he "despised the shame."

for it is the power of God unto salvation to every one that believeth--Here and in Ro 1:17 the apostle announces the great theme of his ensuing argument; Salvation, the one overwhelming necessity of perishing men; this revealed IN THE GOSPEL MESSAGE; and that message so owned and honored of God as to carry, in the proclamation of it, God's own power to save every soul that embraces it, Greek and Barbarian, wise and unwise alike.