Many do not realize that the Latin expression communio sanctorum has two possible meanings. The word sanctorum can be the genitive plural of sancti (“saints,” that is to say, the faithful, as we find it used in St. Paul), and thus it describes the communion among the faithful. It can also be the genitive plural of sancta (“holy things”) and thus designate a communion in holy things (in the sacraments, in grace, in God himself). The grammar of the expression does not allow us to sever these two meanings from each other, since they are both possible meanings of the Latin words. Neither does history allow for this, because both meanings are well attested in the most ancient preaching on the Apostles’ Creed. For faith and for theology, however, there is no doubt: the second meaning alone makes the first possible. It is because Christians commune in holy things, because they gather around the sancta, these goods that they hold in common, that they can together form the communion of saints gathered in the Holy Spirit. The good around which friends gather thus holds a central place in the definition of friendship. This notion is decisive because no koinônia is possible among friends without a common or shared good, but it is also important because it shows that as many friendships are possible as there are goods around which friends gather.
Jean-Pierre Torrell, Christ and Spirituality in St. Thomas Aquinas, ed. Matthew Levering and Thomas Joseph White, trans. Bernhard Blankenhorn, vol. 2, Thomistic Ressourcement Series (Washington, D.C.: The Catholic University of America Press, 2011), 48–49. This book is available free through the end of the November!