St. Augustine notes this and it is used in the Council of Trent: "God commands not impossibilities , but, by commanding, both admonishes thee to do what thou art able, and, to pray for what thou art not able to do."
Also stated is that, "God does not abandon the just without previously having been abandoned by them."
To simplify this introduction to the meaty bits, one can say this. God gives all of us the grace necessary for our salvation. All have sufficient grace for salvation.
So, many people chose not to cooperate with grace. Why would anyone do this?
The predominant faults we have cause us not to cooperate with grace. Pride, greed, envy, lust and so on keep one from following the urgings of grace. Do people actually ignore little hints, or even big hints?
Yes. A man I know was in a horrific accident between his small sport car and a huge truck. He walked away with no injuries. He was living a life away from God as a fallen-away Catholic. Did he mend his ways? No, and in fact, he became a Unitarian.
Another man I know was in a horrible accident as well which would have killed anyone not protected by God. He did not stop being an agnostic and did not see the brush with death as a warning.
Cooperating with grace means reflecting and listening to one's angel, to God, maybe even Our Lady.
Cooperating with grace implies obedience and humility. Resisting grace implies pride and a stubbornness to continue on one's way.
God does not test us nor present us with difficulties beyond our capabilities.
But, He does allow us to become holy in trials.
"...that some are saved is the gift of Him who saves."
to be continued...