Whether There Can Be Sufficient Grace Which Is Not Efficacious?
“No,” if we are talking about actual grace or God’s transient help to act, because God’s actions always attain their end. There is a caveat, however. God can assist a person successfully by touching at least one of the will, the intellect, or the bodily powers of action. Thus,
if God has not influenced the will, then even after the grace is bestowed, one may change his mind and fail to choose the end;
if God has not influenced the intellect, then one may fail to know how to attain the end;
if God has not influenced the powers, then one may fail in his efforts to attain the end.
But actual grace can move a person to action infallibly, if God so wishes.
“Yes,” if the grace is habitual or sanctifying (the permanent state of grace), because that grace is (1) an upgraded nature (the “light of grace”) coming complete with (2) the habits to guide the workings of that new nature, that is, infused virtues and gifts. Grace is necessary, because one has the powers and the disposition to act to merit salvation which he would not have without grace; it can be sufficient, because normally no actual grace is required to move one to do supernaturally meritorious works; yet be inefficacious in that one can at any time go against the disposition and sin and even lose the divine habit altogether. Having grace does not guarantee, though it predisposes one to, supernaturally meritorious acts, just as having any good habit is not always guaranteed to elicit a virtuous act.