Objection 1. It would seem that Christ could not merit for others. For it is written (Ezekiel 18:4): "The soul that sinneth, the same shall die." Hence, for a like reason, the soul that meriteth, the same shall be recompensed. Therefore it is not possible that Christ merited for others.
Objection 2. Further, of the fulness of Christ's grace we all receive, as is written John 1:16. Now other men having Christ's grace cannot merit for others. For it is written (Ezekiel 14:20) that if "Noah and Daniel and Job be in the city [Vulgate: 'the midst thereof']... they shall deliver neither son nor daughter; but they shall only deliver their own souls by their justice." Hence Christ could not merit anything for us.
Objection 3. Further, the "reward" that we merit is due "according to justice [Vulgate: 'debt'] and not according to grace," as is clear from Romans 4:4. Therefore if Christ merited our salvation it follows that our salvation is not by God's grace but by justice, and that He acts unjustly with those whom He does not save, since Christ's merit extends to all.
On the contrary, It is written (Romans 5:18): "As by the offense of one, unto all men to condemnation; so also by the justice of one, unto all men to justification of life." But Adam's demerits reached to the condemnation of others. Much more, therefore, does the merit of Christ reach others.
I answer that, As stated above (8, 1,5), grace was in Christ not merely as in an individual, but also as in the Head of the whole Church, to Whom all are united, as members to a head, who constitute one mystical person. And hence it is that Christ's merit extends to others inasmuch as they are His members; even as in a man the action of the head reaches in a manner to all his members, since it perceives not merely for itself alone, but for all the members.
Reply to Objection 1. The sin of an individual harms himself alone; but the sin of Adam, who was appointed by God to be the principle of the whole nature, is transmitted to others by carnal propagation. So, too, the merit of Christ, Who has been appointed by God to be the head of all men in regard to grace, extends to all His members.
Reply to Objection 2. Others receive of Christ's fulness not indeed the fount of grace, but some particular grace. And hence it need not be that men merit for others, as Christ did.
Reply to Objection 3. As the sin of Adam reaches others only by carnal generation, so, too, the merit of Christ reaches others only by spiritual regeneration, which takes place in baptism; wherein we are incorporated with Christ, according to Galatians 3:27, "As many of you as have been baptized in Christ, have put on Christ"; and it is by grace that it is granted to man to be incorporated with Christ. And thus man's salvation is from grace.
Source: Summa Theologica (New Advent)