[People often say], “Christ’s atonement was sufficient for all, but efficient only for some.” What does this mean? The Calvinist would interpret this axiom to mean that the value of Christ’s sacrifice is so high, His merit so extensive, that its worth is equal to cover all the sins of the human race. But the atonement’s benefits are only effective for believer’s, the elect. The non-Calvinistic interprets this axiom in slightly different terms: Christ’s atonement was good enough to save everyone – and was intended to make salvation possible for everyone. But that intent is realized only by believers. The atonement is efficient [or “works”] only for those who receive its benefits by faith.
R.C. Sproul