Wiktionary
swear by
vb. (context idiomatic English) To wholeheartedly trust.
Usage examples of "swear by".
But if you help us get out, I'll help you, I swear by all the furies.
I swear by my soul, I would never have done such a thing had I been able to choose my own course.
I swear by Apponel, theres no way they can be behind us, and they are anyway.
Each candidate will face as closely as possible in the direction of his home and swear by his own individual God as he repeats after me.