It would not be a sin against faith to doubt an approved apparition. But I would argue that it is a sin against prudence.
The parish priest, the local bishop, the appropriate ecclesiastical congregations in Rome, the Holy Father… they have researched the case to no ends, they have interviewed everyone involved from the visionary down to the old man that was walking his dog by the park one day. They’ve analysed the messages and put it through the wringer and they have all came to the conclusion that it is worthy of belief. A Mass is even written in honour of the apparition such as Our Lady of Fatima.
Sure Catholics don’t have to believe… but they’d be rather imprudent not to. I cannot think of a single reason for a man to doubt an approved apparition that our Holy Mother Church has declared worthy of belief.
It puts ones own judgement above that of the Church.
