Baptism is, in New Testament times, necessary for salvation. Scripture is clear on this. However, from the early centuries, the Church has recognized that baptism need not involve water--the martyr is baptized by blood even if she has not been baptized with water. This idea has been generalized into the notion of a baptism of desire. Someone who wants to be baptized but has been unable to receive the sacrament (e.g., because she is imprisoned apart from anybody willing to baptize her) is incorporated into the mystical body of Christ through her desire (when? at the hour of death? at the time when she desires it? I don't know).
A later development is that of an implicit desire for baptism (see this article by Cardinal Dulles). One philosophical difficulty, however, is in making precise sense of an "implicit" desire. One approach is to use counterfactuals. George implicitly desires baptism if it is the case that were George fully informed, he would desire baptism. This approach, however, seems to require Molinism to work if what we desire is in part dependent on our free choices. Besides, this suffers from many of the standard problems that come up in the case of hypothetical desire satisfaction accounts of welfare.
The better approach is to say that George implicitly desires baptism provided that he actually desires baptism but under some relevantly close other description. If memory serves, me this is the approach Msgr. Van Noort uses to account for the possibility of the salvation of the heathen in his superb Dogmatic Theology, though I do not recall his developing it with sufficient theoretical detail.
The problem now is of what counts as a "relevantly close" description. Van Noort's example, if memory serves, was of the non-Christian who concludes that there is a God and that he is a sinner, who is sorry for his sins and who desires God's means of forgiveness, trusting that God has such means. Unbeknownst to him, baptism is God's means of forgiveness, and so he desires baptism.
"God's means of forgiveness" is a sufficiently relevantly close description of baptism. But it does not seem true that any description will do. Suppose George, on a whim, desires to have happen to him the events described on page 113 of some random book he sees on a shelf but has never opened, so he has no idea of what is on page 113. That book happens to describe a baptism on page 113. Plausibly, that description doesn't count as relevantly close (though we could also imagine George having a religious experience that tells him that what is on page 113 is desirable, and then there might be relevant closeness, though the description will shift: what he really wants to have happen to him are "the events described on page 113 as recommended to him by God"). One reason, maybe the reason, that that description doesn't count as relevantly close is that no element of faith, hope or love need be involved if that is the description. It is just an accident--at least as regards his will (Providence can never be discounted)--that the object of desire is identical with baptism. As far as his will goes, he might as well have whimsically desired to have happen to him what is described on page 187, which let us suppose is a Satanic ritual.
So on this account, the problem of implicit desire for baptism is the problem of closeness of description. This is a problem that comes up in other contexts--it comes up in the context of love (do I really love Patrick if I "theoretically" love the smartest person in New York and Patrick is the smartest person in New York) and of double effect (if I intend to kill the first mammal I see in the zoo, and the first mammal I see and kill in the zoo is the zookeeper, did I intentionally kill a human being?) The problem of closeness of description is difficult in all of these contexts. But the fact that the problem comes up in other contexts suggests that we should not abandon the implicit desire account just because of this problem.
My earlier mention of faith, hope and love is suggestive. Desiring baptism under some descriptions is tied to faith, hope and love. Desiring it under others is not. Maybe it's not so much a question of the content of the description as of the spirit in which one desires. What makes a description relevantly close may be that it is a description of desire such that one is desiring under the description in faith, hope and love. It is necessary that the description in fact be a true description of baptism (or maybe something close enough?), but closeness is measured not in terms of content. Can such a solution be given to the other two closeness problems?