swear by
swear by {v.}
1. To use as the support or authority that what you are saying is truthful; take an oath upon.
A witness swears by the Bible that he will tell the truth.
In ancient Greece a doctor swore by Apollo, the god of healing, that he would be a good doctor.
John swore by his honor he would return the bike.
2. To have complete confidence in; be sure of; trust completely.
When John has to go somewhere fast, he swears by his bike to get there.
We can be sure that Fred will come on time, since his friend Tom swears by him.
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Source: A Dictionary of American Idioms
