The inclusion of True Faith is primarily to up power adversaries. Everything the wizards do is, at least through 4th ed, opposed by the church as presented in the game, and opposed by the historic 12 and 13th century Church. If one is looking for Christian morality as the basis of play, Ars Magic as a mechanical setup makes for a poor choice, as the mechanics are optimized for behaviors that are opposed by the historic church and the in-rulebook church-as-described.
Testament, that the OP said was the type they are looking for, is not based on New Testament christian morality but slightly fantasy/mythical Ancient Israel as a setting with specifically ancient Jewish models as a basis of play with their judge class and such.
Not to mention that, despite appearing to be aimed at villains, the 3rd ed supplement The Maleificum is very much a player supplement as well.
There is also 3rd edition's
Pax Dei the specifically Church Dominion counterpart to the Maleficium.
"Pax Dei is the companion volume to the Ars Magica supplement, The Maleficium. This book delves into Mythic Europe's Church and Divinity, just as The Maleficium explores the milieu of Hell, and its residents' machinations. The two books conjoin and oppose one another, reflecting both sides of the Christian coin."
"Despite the attitude of many Magi, who do not believe in Heaven or Hell but instead look forward to transcendence in Twilight, there are some (especially among post-Christian Roman Magi) who venerate Church and God. There are three ways for Hermetic Magi to come to God. The first is the easiest to name but difficult to come by: True Faith. A Magus who achieves True Faith interacts closely with the Divine, even if she does not possess the Piety Ability. The second path is found through prayer and tithing to Saint Nerius, Patron Saint of the Order of Hermes. By this devotion a Magus may receive grace and be allowed into the family of the Church, maybe even permitted salvation. The third way for Magi to come to God is by becoming what are known as Pious Magi.
The first path to God, True Faith, is discussed in Chapter Three of this book. The second path, devotion to Saint Nerius, is discussed in Chapter Five, under the veneration of saints in general, but more information on Nerius himself is provided below. The third and most dramatic means to God, dedication to pious magic, is discussed here."
Fourth edition had more books for playing outside the normal hermetic magi base including a whole sourcebook on Mythic Judaism Kabbalah and
Ultima Thule for Scandinavian pagan characters.
The base play of Ars Magica is not designed around Christian morality as the basis of its default play, but it is a fairly christian setting with options to incorporate those who want to use Christian morality as the basis of play.