So. Say I have a magus Bonisagus who has two traits. He is deeply interested in astrological magic (and has developed Planetary Magic). And more unusually, he actively wants to unlock and reveal the relevant knowledge to the Order - in short, he wants to integrate Celestial Magic as a regular Hermetic Virtue. Aside from the Bonisagus Oath, he's simply a generous sort with his knowledge (the fact that such a breakthrough might well bring him to the notice of the archmagi is a side benefit, of course).
Aside from joining a Mystery Cult (which almost certainly comes with the baggage of a Vow of secrecy, which would defeat the point of his quest), what paths are open for him to seek this knowledge?
There's searching and gathering musty tomes with appropriate Cult Lore to practice and do self-initiations from (see LoH p.49 box Borrowing Cult Lore and Initiation Scripts).
And there's finding ghosts with the appropriate knowledge and induce them to teach and initiate you.
There's even raiding a book and script depot of the Children of Hermes (TMRE p.124ff), which the Order will certainly approve ...
Your Bonisagus might turn into a Seeker (HoH:TL p.15) still 8) .
The other option is simply doing original research, at least secure in the knowledge that what you want to do is possible...
You can also induct yourself in virtues that you don't yet have. This is, quite simply, really really hard. But, if you develop a cult lore all of your own based around astrological magic, if you write initiation scripts for yourself (this is also really hard), then you can give yourself celestial magic.
Of course, making up your own cult lore is fiendishly hard as ou need to spend seasons spending piddly amounts of practice xp to get it up, but for a character like this you might want to look into it.
Those aside, there's also the "work with your GM" option to establish what sort of Sources of Insight exist that could provide interesting stories for Integration research. Just off the top of my head:
- Go fly up to the Celestial Sphere (or as close as you can get) and take a look at it.
- Summon up a Daimon of Mercury, Venus, Mars, Jupiter, or Saturn, (or one of the signs of the Zodiac, I suppose) and ask politely.
- Find a Source of non-hermetic Astrological lore (https://en.wikipedia.org/wiki/Astrology has some texts listed)
The virtue can be granted by an entity that has the power of teaching it.
An angel might be able to do this, but is your character worthy of such a reward or burden?
Various entities of the Magic Realm might be able to this; the Ritual Power to grant virtues is canonical, but since this tends to cost Might, is your character ready to research these elusive beings and pay their price?
And, for a special low price just for you, various demons will almost certainly arrange for you to learn what you need, especially if you intend to spread your new insights through the Order. How could you possibly lose?
Of course, you can achieve this virtue through the right kind of personal experience. For example, surely astrological virtues seem appropriate for venturing into the Magic Realm for a pilgrimage to meet each of the planetary daimons... but how can you be sure?
Still, doing stuff like this is likely to provide Insight for a Breakthrough.
Or of course you can simply try to investigate the mystery cults for sources of insight. recruit agents, acquire source material, try not to get killed... use these as insights the same way you would for ancient magics...