The highly esteemed theologian J.I. Packer forthrightly addresses in a Christianity Today article the question whether Christian theology allows that others besides Christians might be granted eternal life.
This is a very difficult question for Christians to discuss with devout members of other faiths because the orthodox answer seems heartless and parochial. Packer doesn't shrink from it, however, and states clearly that:
This is the mainstream evangelical position and is the heritage of two thousand years of orthodox Christian belief on the matter. Even so, I think it fair to say that everyone, including Dr. Packer, might hope that this traditional interpretation is incorrect. It breaks one's heart to think of the implications of its being right. Millions of wonderful people who deeply love God as they understand him are nevertheless lost forever if it is. Either they spend eternity suffering the torments of hell or they are completely annihilated, but in either case, Christians who are called by Christ to love the lost cannot contemplate that fate without hoping that it's not so, without hoping that God has some alternative plan for the millions of children and adults who have lived their lives without ever hearing the gospel or who have, for whatever reason, never been able to perceive its truth.
Yet whatever such a plan might be (C.S. Lewis addresses this very issue, albeit obliquely, in his wonderful short work The Great Divorce), it must be consonant with the Biblical witness on the matter, especially the doctrine that it is only because of the price that God himself paid on the cross that anyone at all can be saved. Salvation is possible for anyone who receives it only because of what Christ did, but whether it also depends upon a person's knowing what Christ did and knowing who Christ was is less clear.
Packer goes on to outline two alternatives to the orthodox view that Christians have put forward throughout the history of the church and finds them both inadequate:
We discount universalism for reasons we need not go into now, and we agree with Dr. Packer that if any non-Christians are saved it is not on the basis of sincerity, merit, or rituals. However, it may be that these do not exhaust all the possibilities. Most Christians, after all, believe that children and mentally handicapped persons who die without recognizing themselves as "guilty, defiled, and unworthy," and confessing and renouncing their sins, "asking mercy from whatever gods there may be," are nevertheless not condemned by their lack of understanding.
Many Christians also believe that people who lived before the Christian era are also held to a different standard and are not necessarily denied salvation even though they may never have had the faintest glimmer of who Christ would be. So, it seems reasonable to hope that it is at least possible that adults in the present age who've never heard the gospel or who, for reasons transcending the biblical "hardness of heart," find themselves unable to discern its truth, are judged on a quite different basis than those who have heard the gospel and who have no other reason for rejecting it than that they simply don't want it to be true.
The gospel tells us in many places that all those who accept Jesus Christ as their Lord and savior will have eternal life, but it rarely suggests that only those who accept Christ will be saved. In those passages (like Jn.8:24, I Jn.2:22,23 and I Jn. 5:10) which do seem to imply this exclusivism it is possible to read the text as referring to those contemporaries who have been given so much evidence that Jesus was the Son of God that their persistent unbelief is without excuse.
Perhaps when each of us stands before God he will ask of us a single question. Perhaps he will ask not whether we have believed this or that point of doctrine, however important these may be, but rather he will ask of each person: "Do you love me?" It could be that this is for God the all-important issue, the overriding question. Our love is what he most earnestly desires. It could be, too, that we will not respond to this question in words, but rather that our whole life will serve as our reply.
On the other hand, it may well be that this is not the case at all and that Packer's interpretation of the Gospel's teaching is correct, but he, and every other Christian, should be fervently hoping that it isn't. We should all hope that there's a "wideness to God's mercy" and grace that judges us accountable for the love of God, or lack of it, that we hold in our heart and not for the knowledge of the scripture, or lack of it, which we hold in our head.
