I tend to agree with Lief Erikson.
On the religion threads on Amazon, the atheists and agnostics will ridicule and belittle the Bible as little more than a comic book of ancient fairy tales, irrational stories and claims, and tenets of the faith that defy logic. Inevitably, some Christian, usually of the Pentecostal or Evangelical persuasion, will try to refute their post by citing the Bible, the very book they claim is laughable rubbish. And then the feeding frenzy of ridicule of that Christian poster begins. It’s sad.
I might suggest that you tread lightly using Marian miracles and apparitions as proof of our beliefs in Mary. It’s a little like using the Bible to prove something to people who don’t believe in the Bible. After all, you may find no enjoyment in watching a football game, and it would be foolish of me to try to prove how wonderful football is by explaining the halfback option pass to you.
When talking about Mary, I always state that our beliefs, though they are extra-biblical, are not necessarily anti-Biblical. God glorifies man by coming to us as one of us, through one of us. And, the position of the Church when it “approves” an event such as Fatima is stating that it has investigated it, and finds that there is nothing contrary to the deposit of the faith. Thus, Catholics are free to believe in and have devotion to an event, but no one is obligated to do so. It is a somewhat ‘negative’ endorsement. The Church doesn’t say the event is ‘true’, it is saying that there is nothing in opposition to the faith in it. (the only exception of course is the doctrine of the Assumption of Mary that was pronounced a tenet of the faith in 1950 by Pope Pius in an infallible ‘ex cathedra’ pronouncement)
A good thought several teachers I have encountered promulgate is the idea of ‘being open to the possibility’ of a tenet of faith. Rather than trying to ‘prove’ our beliefs, present Marian devotion well, and simply state that we all always open to the possibility of God’s intervention in our lives, through whatever means he chooses. And we see much of that intervention in his choice to come to us in the mother of His Son, in Bethelehem, in Nazareth, on Calvary, and through time.
Good luck, God Bless