An act or omission in which a person tries to test God's attributes, notably his love, power, or wisdom. It may be done either explicitly or implicitly.
God is tempted explicitly when something is demanded of him or when something is done or omitted for the actual purpose of ascertaining the extent of god's love, power, or wisdom. An example would be that of an atheist who might give God, say, five minutes to strike him or her dead, "if there is a God." To explicitly tempt God is always a grave sin.
God is tempted implicitly when, not doubting his attributes, a person rashly requires a manifestation of divine love, power, or wisdom. Examples of implicitly tempting God are needlessly risking one's life in some dangerous feat or refusing all medical aid simply because it is up to god to cure one of some grave illness or disease. Implicitly tempting god is a serious sin in grave matters, otherwise a venial sin.