I think grace may be something that is shown to us by God in increments. Not that I'm suggesting the spiritual equivalent of quantum mechanics, but I do think divine light dawns upon the human spirit in measures. If that light has the intended effect, then more light is shone. If it doesn't, then what light had been shining is withdrawn.
When "grace" (or "light") is used in this manner, what it really is referring to is the action of the Spirit of God. Grace, I think, is a term which is used, generally, very inaccurately in the Church. When it is used, more often than not, it conjures up a picture of some mystical force or power flowing from God unto that which he has made. Grace is not such a force or power, it is merely a sentiment in the heart of God toward that which he has made--joyful kindness, unmerited favor.
It is the Holy Spirit (or, at times, those ministering spirits called angels) that actually reaches out and touches someone or something with the application of light and power. God's grace actually does nothing, but God by grace surely does! Nonetheless, in keeping with the way in which grace is used broadly (even if erroneously), I use it here to refer to the unseen activity of the Spirit by which spiritual qualities are imbued to the spirit/soul of one who is a believer or one who may become a believer. In other words, grace is shorthand for the work of the Holy Spirit.
I do not believe that the seed of Adam is capable of receiving such grace unmitigatedly. The darkened souls of Adam's sinful race would be overwhelmed if that were to occur, and the effect would be to blight the image of God they still possess. The image of God entails freewill by necessity and God's design, and it is not God's will to emasculate or eradicate the independence that comes thereby. Therefore, God's drawing, convicting grace does not and cannot come upon man as a storm surge, irresistibly, or it would incapacitate the image of God within.
Grace, it seems to me, is measured out by incremental nudges.
Light shines in some measure upon the souls of men. The soul so illumined which responds to that light gets more light, and softens. The soul repulsed by that light remains in darkness and hardens. The journey of the faithful from rank unbelief to oneness with God is one of responding to increasing grace and brightening light.