Objection 1. It seems that the sacraments of the New Law do not contain grace. For it seems that what is contained is in the container. But grace is not in the sacraments; neither as in a subject, because the subject of grace is not a body but a spirit; nor as in a vessel, for according to Phys. iv, "a vessel is a movable place," and an accident cannot be in a place. Therefore it seems that the sacraments of the New Law do not contain grace.
Objection 2. Further, sacraments are instituted as means whereby men may obtain grace. But since grace is an accident it cannot pass from one subject to another. Therefore it would be of no account if grace were in the sacraments.
Objection 3. Further, a spiritual thing is not contained by a corporeal, even if it be therein; for the soul is not contained by the body; rather does it contain the body. Since, therefore, grace is something spiritual, it seems that it cannot be contained in a corporeal sacrament.
On the contrary, Hugh of S. Victor says (De Sacram. i) that "a sacrament, through its being sanctified, contains an invisible grace."
I answer that, A thing is said to be in another in various ways; in two of which grace is said to be in the sacraments. First, as in its sign; for a sacrament is a sign of grace. Secondly, as in its cause; for, as stated above (Article 1) a sacrament of the New Law is an instrumental cause of grace. Wherefore grace is in a sacrament of the New Law, not as to its specific likeness, as an effect in its univocal cause; nor as to some proper and permanent form proportioned to such an effect, as effects in non-univocal causes, for instance, as things generated are in the sun; but as to a certain instrumental power transient and incomplete in its natural being, as will be explained later on (4).
Reply to Objection 1. Grace is said to be in a sacrament not as in its subject; nor as in a vessel considered as a place, but understood as the instrument of some work to be done, according to Ezekiel 9:1: "Everyone hath a destroying vessel [Douay: 'weapon'] in his hand."
Reply to Objection 2. Although an accident does not pass from one subject to another, nevertheless in a fashion it does pass from its cause into its subject through the instrument; not so that it be in each of these in the same way, but in each according to its respective nature.
Reply to Objection 3. If a spiritual thing exist perfectly in something, it contains it and is not contained by it. But, in a sacrament, grace has a passing and incomplete mode of being: and consequently it is not unfitting to say that the sacraments contain grace.
Source: Summa Theologica (New Advent)