I don't have any ready-made adventures on the topic, but it's an interesting idea, so here's some advice (for what it is worth).
Ars Magica 5 has, as one of its core ideas, that the Divine is the ultimate power and the Bible is true. Now, there is a little hand waving when it comes to the related faiths of Judaism and Islam, which are also true, even when they contradict. This is part of the mystery of God.
This decision, to make the divine story of creation true, contrasts to other fantasy games and with many of our secular players. Quite simply, many players do not want to play characters who are religious. They may consider religion to be irrational. For whatever reason, the player wants to make a character who is an atheist.
In many role playing games, being an atheist is a perfectly reasonable personality trait. In Mythic Europe, the setting for Ars Magica, being an atheist is irrational. The power of the divine is measurable. Angels appear with regularity. Saints can be invoked. The power of the Divine trumps Hermetic magic.
Now, that doesn't mean the divine story of creation has to be true in your campaign. It is in the official Ars Magica setting. But you would not be the first group to decide otherwise. It might be worth your time to think about the direction you want your game to take.
But, to get back to your original question, if you want to do a story in which the creation is proven to be true, you could easily introduce an angel who was a witness to it. But the magi may still refuse to believe such an individual. In truth, if they don't believe all the other evidence in the Divine that is already available to them, why would they believe one more piece?
It may be more interesting, if your player characters insist on denying basic truths about the nature of the universe, to throw them into stories where that denial is interesting: a debate with a Franciscan friar, for example; an encounter with a village which is grateful to the magi for their aid but who throw the magi out for their obviously heretical views; the threat of military action by Pope-sanctioned forces, determined to wipe out the heresy.