Upwards of 70,000 people witnessed the miracle of the sun at Fatima, including some atheistic Communists who would not likely be taken by a mass hallucination. So clearly something real happened. Now you believe that the devil pulled this off somehow, when God could not have?
So the devil is somehow capable of getting into the minds of 70,000 people and causing every single one of them to see the same hallucination? That's unlikely. Clearly something physical took place outside of people's minds. This was no illusion. If it was the work of the devil, he would have to have done something that looked like the sun coming down from the sky and also at the same time dried the ground and people's clothes. Perhaps he had a legion of demons flying around with invisible hair-dryers drying everything off while a few of them brought some fireball down from the sky.
It stretches credibility to its absolute limit to attribute this to the devil.
And then of course there's the fact that the Church has clearly and broadly approved of Fatima. You can find imprimatur after imprimatur after nihil obstat giving approval to books written about Fatima and the Fatima message, of widespread support from the Church for making the First Saturdays, etc.
Bishop José Alves Correia da Silva declared the events worthy of belief on 13 October 1930. On 13 May 1946, Pope Pius XII granted a canonical coronation to the venerated image enshrined at the Chapel of the Apparitions of Fátima via his apostolic legate, Cardinal Benedetto Aloisi Masella. On 11 November 1954, he raised the Sanctuary of Fátima to the status of a minor basilica by his papal brief Lucer superna.