Consequently, if what a
man asks for will not tend to his ultimate attainment of God, he
does not merit it by his prayer; sometimes, indeed, by asking
and desiring such a thing he may lose all merit, as, for
example, if a man were to ask of God something which was sinful
and which he could not reverently ask for. Sometimes, however,
what he asks for is not necessary for his salvation, nor yet is
it clearly opposed to his salvation; and when a man so prays he
may by his prayer merit eternal life, but he does not merit to
obtain what he actually asks for. Hence S. Augustine says[239]:
"He who asks of God in faith things needful for this life is
sometimes mercifully heard and sometimes mercifully not heard.
For the physician knows better than the patient what will avail
for the sick man." It was for this reason that Paul was not
heard when he asked that the sting of the flesh might be taken
away--it was not expedient. But if what a man asks for will help
him to the attainment of God, as being something conducive to
his salvation, he will merit it, and that not only by praying
for it but also by doing other good works; hence, too, he
undoubtedly will obtain what he asks for, but when it is fitting
that he should obtain it: "for some things are not refused to us
but are deferred, to be given at a fitting time," as S.
Augustine says.[240] Yet even here hindrance may arise if a man
does not persevere in asking; hence S. Basil says[241]: "When
then you ask and do not receive, this is either because you
asked for what you ought not, or because you asked without
lively faith, or carelessly, or for what would not profit you,
or because you ceased to ask." And since a man cannot,
absolutely speaking, merit eternal life for another, nor, in
consequence, those things which belong to eternal life, it
follows that a man is not always heard when he prays for
another. For a man, then, always to obtain what he asks, four
conditions must concur: he must ask for himself, for things
necessary for salvation; he must ask piously and perseveringly.

3. Lastly, prayer essentially reposes upon faith, as S. James says: _But
let him ask in faith, nothing wavering_.[242] But faith is not
sufficient for merit, as is evident in the case of those who have faith
without charity. Therefore prayer is not meritorious.