In the man Jesus Christ however there was no movement of sense that was not controlled by reason; and even his natural bodily activities were in a sense voluntary, inasmuch as that he willed that his flesh should do and suffer according to its own proper nature. So there is even greater unity of activity in Christ than there is in other men. … Christ’s grace was not just his own personal grace but the grace proper to the head of the whole Church, to whom all members are joined so as to constitute one person mystically. So what Christ earned he earned for all his members, just as what man does with his head serves all his members. The sin of Adam, whom God appointed to beget the whole of humanity, passed on as inheritance to others by bodily propagation; the earnings of Christ, whom God set up as the head of all men by grace, pass on to all his members by spiritual birth of baptism which makes us members of Christ’s body.
Saint Thomas Aquinas Summa Theologiae Part III, chapter 13, 19.1
