St. Thomas Aquinas
“This sacrament has a threefold significance.
One with regard to the past… it is commemorative of our Lord’s Passion, which was a true sacrifice… and in this respect it is called a ‘Sacrifice.’
With regard to the present… that of Ecclesiastical unity, in which men are aggregated through this Sacrament; and in this respect it is called ‘Communion.’
With regard to the future… 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.”
Source: St. Thomas Aquinas, Summa Theologica