And there are three things required in religion for worship of God, that is, faith, hope, charity. In faith, what must be believed; in hope, what must be hoped; in charity, what must be loved. Faith ("fides") is that whereby we truly believe what we are not able at all to see. For believing is of what we are not able to see. And 'faith' is said in the proper sense if everything is done ("fiat") that is said or promised. And it is called faith, from the fact that what has been done ("fit") is that which was agreed between two, as between God and man; therefore it is also covenant ("foedus"). That is called hope ("spes") which is a foot going forward ("pes progrediendi"), as in "It is a foot" ("est pes"). Thus also for the contrary desperation: the foot is lacking ("deest pes") there, and there is no ability to go forward, because as long as someone loves sin, he does not hope for glory. Caritas in Greek and love ("dilectio") is understood as that which binds two in itself ("duos in se liget"). Indeed, love begins from two, which are love of God and neighbor; as the Apostle says (Rm 13:10), "Love is the fullness of law."
[Isidore of Seville, Etymologies, Book VIII, Section 2.]