in the beginning vs to begin with

in the beginning and to begin with both are adverbs.

Word	Noun	Adjective	Verb	Adverb
in the beginning	No	No	No	Yes
to begin with	No	No	No	Yes

As adverbs, in the beginning and to begin with are synonyms defined as:

Other synonyms of in the beginning include earlier, in the first place, originally.

in the beginning (adverb)	to begin with (adverb)
with reference to the origin or beginning	before now
before now