97 Wherefore, since the chief obstacle to this study is the power of unbridled lust, which indeed is the most potent cause of sinning against the sacred laws of matrimony, and since man cannot hold in check his passions, unless he first subject himself to God, this must be his primary endeavor, in accordance with the plan divinely ordained. For it is a sacred ordinance that whoever shall have first subjected himself to God will, by the aid of divine grace, be glad to subject to himself his own passions and concupiscence; while he who is a rebel against God will, to his sorrow, experience within himself the violent rebellion of his worst passions.
Source: Casti Connubii (Vatican.va)