in truth vs truly

in truth and truly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
in truth	No	No	No	Yes
truly	No	No	No	Yes

As adverbs, in truth and truly are synonyms defined as:

in truth and truly: in fact (used as intensifiers or sentence modifiers)

Other synonyms of in truth include really.

in truth (adverb)	truly (adverb)
in fact (used as intensifiers or sentence modifiers)	in accordance with truth or fact or reality
	by right
	with sincerity; without pretense
	in fact (used as intensifiers or sentence modifiers)