“This sacrament has a threefold significance. one with regard to the past, inasmuch as it is commemorative of our Lord’s Passion, which was a true sacrifice, as stated above (Q
[48], A[3]), and in this respect it is called a ‘Sacrifice.’ With regard to the present it has another meaning, namely, that of Ecclesiastical unity, in which men are aggregated through this Sacrament; and in this respect it is called ‘Communion’ or {Synaxis}. For Damascene says (De Fide Orth. iv) that ‘it is called Communion because we communicate with Christ through it, both because we partake of His flesh and Godhead, and because we communicate with and are united to one another through it.’ With regard to the future it has a third meaning, inasmuch as this sacrament foreshadows the Divine fruition, which shall come to pass in heaven; and according to this it is called ‘Viaticum,’ because it supplies the way of winning thither. And in this respect it is also called the ‘Eucharist,’ that is, ‘good grace,’ because ‘the grace of God is life everlasting’ (Rom. 6:23); or because it really contains Christ, Who is ‘full of grace.'” -St. Thomas Aquinas