backslide vs lapse

backslide is not a noun while lapse is a noun.

Word	Noun	Adjective	Verb	Adverb
backslide	No	No	Yes	No
lapse	Yes	No	Yes	No

As verbs, backslide and lapse are synonyms defined as:

backslide and lapse: drop to a lower level, as in one's morals or standards

backslide (noun)	lapse (noun)
	a mistake resulting from inattention
	a failure to maintain a higher state
	a break or intermission in the occurrence of something

backslide (verb)	lapse (verb)
drop to a lower level, as in one's morals or standards	go back to bad behavior
	drop to a lower level, as in one's morals or standards
	end, at least for a long time
	for time to move forward
	pass into a specified state or condition; sink into
	let slip