So your companion character with True Faith, who is member of a holy tradition, can learn the Supernatural Abilities associated with it quite easily, and doesn't need the respective Virtues for them.
This does not imply, that a companion level character with True Faith has already learned all the Supernatural Abilities of her tradition at the onset of the saga. Best ask your troupe for a decision here - but show them first:
Sure - as a game mechanic, it's quite powerful, in that it (essentially) represents 10 point's worth of major and minor holy virtues, in addition to making additional Holy Virtues easier to learn. (As Divine abilities are usually Difficult, as opposed to Accelerated, you're not penalized nearly as much for trying to learn new ones.) Of course, casting methods are probably not all that fun as a major virtues go, so it's ESSENTALLY one full virtue's worth of guidelines for the cost of 3.
But if you've got access to additional sources of Holy Magic outside of your 3+1 freebies (such as Kaballah, which potentially knows all of them, somehow), then yeah - it's a pretty good deal.
That being said, it's (arguably) one of the most character-defining virtues out there; if PC's are taking it for the point break, that's probably missing the point, as well as missing all of the associated behaviors that would likely go into this. People with True Faith tend to dedicate their lives to promoting that Faith - either externally (such as St. Francis of Assisi) or Internally (by going off and becoming monks or holy men in the wilderness.) Any Magi or Companion taking True faith should really be roleplayed with that in mind.
For example, the Jewish magi character I have right now has True Faith - and his whole thing is about setting up a Jewish homeland (or at least refuge) for Jews around the covenant (it's in Iceland - well, it's actually in Colorado, but the portal is in Iceland) for when the next pogrom inevitably happens. And he works on it regularly: getting resources for housing, and setting up meetings with Jewish community leaders across Europe, maintaining an understanding of the political scene in various cities, and so on.
This is only my ruling, but in most campaigns, what I would do is allow a Mythic Companion to take a Holy Tradition and be reckoned as equivalent to a Gifted hedge-wizard - and thus filling up a player's Magus slot.
And I definitely agree about how True Faith is character-defining. If you're playing a character with True Faith, that's as much of a built-in Story Flaw or more than the requirements of the Code built into the Hermetic Magus virtue. I believe I mentioned before in another thread that holy characters should be adventuring almost constantly, and have an excuse to get involved in almost any story where God or the Faith's interests can be defended or advanced.
I would second what others have already mentioned, particularly OneShot, with a few extra comments.
If you have True Faith, you can learn Divine supernatural abilities a) in play b) if someone teaches you. You are not allowed to have them at the start of the game without taking the appropriate Virtues. This means that you'll most likely have to go through some stories that involve your relationship with your would-be mentors; they, not you, decide what you learn and when you learn it.
True Faith is not the only way to gain Divine supernatural abilities after character creation. Initiation, pilgrimages, mystical contemplation are all venues that can grant you access. Yes, with True Faith it is easier; but True Faith comes with its own issues (see below).
True Faith is a very ... encumbering Virtue. It's supposed to be character-defining, and this is partially enforced by the mechanics of Crises of Faith (see RoP:D). They can happen if you stray from the path of True Faith, but they can also happen to a perfectly nice person like Job just because God loves that person in His own unfathomable way. When they happen - zac! - no more divine power for you: all your Virtues tied to the Divine, including all Methods and Powers, stop working until the crisis is resolved (and there's no guarantee it will). As a consequence, perhaps paradoxically, the last two True Faith PCs we've seen in our games had essentially no other major supernatural powers.