The phrase “His grace is sufficient” is a reference the power of his spiritual gifts and endowments to us as we progress through covenants. The grace which is the light of Christ is given to all mankind. That grace imparts light and truth to all mankind. If they will give heed to the grace given them then they will repent and be baptized they can receive that grace which is the gift of the Holy Ghost. As they give heed to that, they can grow into the grace which is the gift of the Holy Ghost until they are sanctified and their garments are washed white in the blood of Christ. As we continue from grace to grace we eventually receive that grace which is the presence of the Son himself, and then even the Father will come and make their abode with them. The point is not that sin we still participate in is bypassed, though yes, sin we are not longer indulging in can be forgiven through the grace of Christ. The point of the phrase “his grace is sufficient” is that he grants spiritual endowments that as we strive to keep our covenants and remove sin from ourselves, he will make it easier to do by granting us more grace. The gift of the Holy Ghost is an example of that grace that shows how it works. The more we have the Holy Ghost the easier is is for us to behave, and the more clearly we know his will and submit to it. But when we reject the grace which is the gift of the Holy Ghost, we are actually worse off than we were before. That is an example of the grace of Christ. It is manifest by covenants that grant us specific spiritual endowments that empower and enlighten us. Those covenants will continue, one after the other past the sealing ordinance all the way until we are one with the Father and the Son in the celestial world.
His grace is sufficient means that by granting us one grace after another in the regular progression it becomes both easier to submit our will completely to the will of the Father, as well as making that will more fully known to us. There is no point in knowing it beyond the point we have grown willing to hearken to it.
The phrase “His grace is sufficient…” could just as well have been rendered “His willingness to grant us spiritual endowments on condition of obedience is sufficient…”