Fides came to mean “faith”, “constant belief” during Christian times, as in a “constant belief in God”. The Roman concept of Fides is more complex and is about another sort of constancy altogether– the constancy of being a reliable, trustworthy person.
To have Fides is to be an ethical partner in relationship with others. It’s related to the later medieval concept of Honor, but is a more socially driven virtue. Honor is to a great degree socially driven, since it’s about doing the right thing according to your station, but it’s also intensely personal. With honor, there is an expectation that you will behave exactly the same no matter how badly the other person or persons is behaving. Honor has an inflexibility to it. You behave with honor because it’s the correct thing to do, not because of the other person.
By contrast, Fides is about mutually respectful relationships. Fides requires society to function at all, and Fides makes society function smoothly. The Goddess Fides oversaw treaties between nations for the Roman people. Perhaps we need her these days, when our communities are so loosely connected, to oversee treaties between individual people and unite them for the common good.