Lapse, relapse, backslide and their corresponding nouns, lapse, relapse, backsliding, are comparable when they mean to fall back into a state or condition from which one has raised oneself or has been raised, or the act or state of one who has so fallen back.
As distinguished from decline, degenerate, and deteriorate, these verbs do not necessarily imply the reversion of a process or development or the gradual losing and the inevitable loss of a valuable quality (as strength, power, or influence) but they do distinctively imply a failure to continue without break a course of improvement and a return, often quickly effected but not always irreparable, to an earlier bad or lower state or condition. Both lapse and relapse basically imply a sliding or slipping but they are increasingly divergent in their applications and connotations.
Lapse usually presupposes reformation in manners, morals, or habits, or the acceptance of a high standard (as of rectitude, accuracy, or accomplishment). It need not imply culpability or weakness, for it often suggests no more than a sudden failure of the memory or the influence of habit or tradition or the pressure of an overwhelming emotion.
When culpability is strongly implied, the word still, in comparison to the other terms in the group, often connotes extenuating circumstances; it is therefore the fitting choice when the context indicates such circumstances.
Relapse presupposes definite improvement or an advance (as toward health or toward a higher physical, moral, or intellectual state) and it implies a complete and often dangerous reversal of direction; thus, one whose improvement in a serious illness has been marked may be said to relapse, or suffer a relapse, when his condition becomes definitely worse; a reformed thief is said to relapse when he returns to his old life.
Backslide and backsliding also imply a reversal in direction of one who has been going forward, but unlike relapse, which is in many ways their close synonym, they are restricted in their reference largely to moral and religious lapses. They therefore often suggest unfaithfulness to one's duty or allegiance or to principles once professed.