St. Thomas Aquinas
“In this sacrament we may consider both that from which it derives its effect, namely, Christ contained in it, as also His Passion represented by it; and that through which it works its effect, namely, the use of the sacrament, and its species. Now as to both of these it belongs to this sacrament to cause the attaining of eternal life. Because it was by His Passion that Christ opened to us the approach to eternal life, according to Heb. 9:15: ‘He is the Mediator of the New Testament; that by means of His death . . . they that are called may receive the promise of eternal inheritance.’ Accordingly in the form of this sacrament it is said: ‘This is the chalice of My blood, of the New and Eternal Testament.’
Source: St. Thomas Aquinas, Summa Theologica