In Ursinus’s Commentary on the Heidelberg Catechism, we are given the explanation for the distinction between the “sufficiency” of the atonement and the “efficiency” of the atonement. Ursinus writes:
Christ satisfied for all, as it respects the sufficiency of the satisfaction which he made, but not as it respects the application thereof; for he fulfilled the law in a two-fold respect. First, by his own righteousness; and secondly, by making satisfaction for our sins, each of which is most perfect. But the satisfaction is made ours by an application, which is also two-fold; the former of which is made by God, when he justifies us on account of the merit of his Son, and brings it to pass that we cease from sin; the latter is accomplished by us through faith. For we apply unto ourselves, that merit of Christ, when by a true faith, we are fully persuaded that God for the sake of the satisfaction of his Son, remits unto us our sins. Without this application, the satisfaction of Christ is of no benefit to us. (pg. 215)
However, David has shown us a different understanding of this. Here is how Herman Witsius used the distinction:
We therefore conclude, 1st. That the obedience and sufferings of Christ, considered in themselves, are, on the account of the infinite dignity of the person, of that value, as to have been sufficient for redeeming not only all and every man in particular, but many myriads besides, had it so pleased God and Christ, that he should have undertaken and satisfied for them. (The Economy of the Covenants, 2.9.2; 1:256)
Do you see the difference? Ursinus says that Jesus death actually is sufficient, whereas Witsius said it would have been sufficient, had God so willed it to be. Witsius teaches a hypothetical sufficiency while Ursinus teaches an actual sufficiency.