Jansenius: what sufficient and efficacious grace mean
Thesis I. Jansenius, bishop of Ypres, and his followers give the name sufficient grace only to a grace after which God need supply nothing further, by way of principle, for a person to will and act. Efficacious grace, for them, is only the grace that does more than give created nature the strength to will: it produces the willing itself in that nature, and of itself bends and determines the faculty of will to will.
On their principles, then, no grace is truly sufficient and yet not efficacious except one that gives the creature the ability to will so fully that no further divine help toward willing is needed beforehand, and that is yet so subject to the creature's freedom that whether to use that grace or not rests with its free choice. Conversely, the only truly efficacious grace is not one that the will's act happens to follow somehow, but one that brings the creature's will under itself, moves it of itself effectively to will, and by its own force infallibly produces the act of willing in it.
Latin
THESIS I. JANSENIUS Episcopus Iprensis, & qui doctrinam illius sequuntur, illam solam gratiam sufficientem vocant, praeter quam nihil aliud ex parte Dei per modum principii necessarium est, ut homo velit & operetur. Gratia vero efficax ea sola illis est, quae naturae creatae non solum dat volendi vires, sed etiam ipsum velle in ea operatur, & volendi facultatem ad volendum ipsa per se flectit & determinat. Adeoque, juxta ipsorum principia, nulla gratia vere sufficiens est, & tamen non efficax, nisi illa quae sic dat creaturae posse velle, ut praeterea nullum divinum auxilium ad volendum praerequiratur; & tamen sic creaturae libertati subdita est, ut penes ejus liberum arbitrium remaneat gratia ista uti vel non uti. Vicissim vero ea sola gratia vere efficax est, non quam quocunque modo sequitur voluntatis actus, sed quae sibi subjicit creaturae voluntatem, eamque ad volendum efficaciter per se movet, ipsum volendi actum vi sua infallibiliter in ea producendo.