Objection 1. It seems that Baptism has not an equal effect in all. For the effect of Baptism is to remove guilt. But in some it takes away more sins than in others; for in children it takes away only original sins, whereas in adults it takes away actual sins, in some many, in others few. Therefore Baptism has not an equal effect in all.
Objection 2. Further, grace and virtues are bestowed on man by Baptism. But some, after Baptism, seem to have more grace and more perfect virtue than others who have been baptized. Therefore Baptism has not an equal effect in all.
Objection 3. Further, nature is perfected by grace, as matter by form. But a form is received into matter according to its capacity. Therefore, since some of the baptized, even children, have greater capacity for natural gifts than others have, it seems that some receive greater grace than others.
Objection 4. Further, in Baptism some receive not only spiritual, but also bodily health; thus Constantine was cleansed in Baptism from leprosy. But all the infirm do not receive bodily health in Baptism. Therefore it has not an equal effect in all.
On the contrary, It is written (Ephesians 4:5): "One Faith, one Baptism." But a uniform cause has a uniform effect. Therefore Baptism has an equal effect in all.
I answer that, The effect of Baptism is twofold, the essential effect, and the accidental. The essential effect of Baptism is that for which Baptism was instituted, namely, the begetting of men unto spiritual life. Therefore, since all children are equally disposed to Baptism, because they are baptized not in their own faith, but in that of the Church, they all receive an equal effect in Baptism. Whereas adults, who approach Baptism in their own faith, are not equally disposed to Baptism; for some approach thereto with greater, some with less, devotion. And therefore some receive a greater, some a smaller share of the grace of newness; just as from the same fire, he receives more heat who approaches nearest to it, although the fire, as far as it is concerned, sends forth its heat equally to all.
But the accidental effect of Baptism, is that to which Baptism is not ordained, but which the Divine power produces miraculously in Baptism: thus on Romans 6:6, "that we may serve sin no longer," a gloss says: "this is not bestowed in Baptism, save by an ineffable miracle of the Creator, so that the law of sin, which is in our members, be absolutely destroyed." And such like effects are not equally received by all the baptized, even if they approach with equal devotion: but they are bestowed according to the ordering of Divine providence.
Reply to Objection 1. The least baptismal grace suffices to blot out all sins. Wherefore that in some more sins are loosed than in others is not due to the greater efficacy of Baptism, but to the condition of the recipient: for in each one it looses whatever it finds.
Reply to Objection 2. That greater or lesser grace appears in the baptized, may occur in two ways. First, because one receives greater grace in Baptism than another, on account of his greater devotion, as stated above. Secondly, because, though they receive equal grace, they do not make an equal use of it, but one applies himself more to advance therein, while another by his negligence baffles grace.
Reply to Objection 3. The various degrees of capacity in men arise, not from a variety in the mind which is renewed by Baptism (since all men, being of one species, are of one form), but from the diversity of bodies. But it is otherwise with the angels, who differ in species. And therefore gratuitous gifts are bestowed on the angels according to their diverse capacity for natural gifts, but not on men.
Reply to Objection 4. Bodily health is not the essential effect of Baptism, but a miraculous work of Divine providence.
Source: Summa Theologica (New Advent)