Musar for Sanhedrin 147:19

Our Rabbis, of blessed memory, said, "If one speaks gossip, it is as though he denied God" (Arakin 15b, and see T.P. Peah, 1:1). As it is said (Ps. 12:15), "Who have said : 'Our tongue will we make mighty : our lips are with us : who is lord over us?' " (Arakin 16b, and see T.P. Peah, 1:1). And therefore our Sages considered him as though he denied God, for he does a great evil to his companion, by making him odious in the eyes of the people and causing him other losses, though he himself derives no profit out of this. There is no doubt that one who makes a habit of speaking gossip has thrown off the yoke of Heaven from upon him, for he sins without any benefit to himself, and he is worse than a thief or an adulterer, for they pursue after their pleasure (Shohar Tov, 120:3). And there is no throwing off the yoke of Heaven as bad as when one deals in gossip. Our Sages further said, "The sin of gossip is weighed equally with the sin of idolatry and sexual immorality and bloodshed" (Arakin 15b). Now it is very astonishing that gossip should be equated with these sins, which are those to avoid which a man must rather let himself be killed than transgress (Sanh. 74a). And they said, "The sin of idolatry is so great that anyone who confesses to it, it is as though he denied the whole Torah" (Hullin 5a). And they said, "One who lusts after any form of idolatry, is a rebel against the whole Torah" (Hullin 4b).