What are the principle causes of grace?

The principal cause of grace is God himself since it’s a participation in the divine nature.

Since grace isn’t a subsistent reality it presupposes a subject in which it begins and then continues the soul in this circumstance of which it’s an accident. Since it’s an accident that's supernatural, it’s drawn forth from the obediential potency of the soul. This potency is its aptitude to receive all that God can will to give it, God can give it anything essentially hence the soul having potency to receive grace and glory to an even higher extent than what was given. It’s formally a passive potency but if it resides in an active faculty then its materially active like when the will receives infused charity. The production of grace presupposes in the adult some movement of the free will as disposition. Even though a repeated good act brings about an acquired habit the disposition treated cannot be caused from grace which is an infused habit. To he who under actual grace does what he can to prepare for justification then habitual grace is given infallibly because it comes from God who moves efficaciously and infallibly.

St. Thomas:

“If God who moves intends that man attain grace, he attains it infallibly.”ST, q. 112, a. 13

In proportion to the person’s disposition he will receive higher or lower degree of grace. But God distributes His gifts more or less profusely so the Church may be adorned with different levels of grace and charity. Special revelation alone can give a man certitude that he’s in the state of grace. Even though we don’t know that we have grace or if God dwells in us, there will be signs.