There are promises.
There are promises kept.
If you don't know someone very well, are they healthy? Do they look like someone who can keep a promise to themselves? Because if one can't keep a promise to themselves, will they keep other promises?

What is the successful promise keeper's instrument?  Its will. As in:

I will.
I will not.

Most are good at one, but not the other.
Being faithful usually is a case of resisting temptation. Not doing.
Going out everyday and providing is doing.

Besides knowing what to do, one has to be willing to do, and not do, in the right measure.