Luther was a devout Catholic priest, at one time (before he went “nuts”).
That opinion must be weighter than RC scholars who have a more objective rational view of him, nor will that explanation do, since the 1520 quote (see here ) you enlist was during the same period that he refused to retract all of his writings at the demand of Pope Leo X in 1520 and the Holy Roman Emperor Charles V at the Diet of Worms in 1521.