According to my stats, somebody reached this site after searching for ‘catholicism causes atheism’. You’d think. From the Wikipedia article on transubstantiation:
A hat’s shape is not the hat itself, nor is its colour the hat, nor is its size, nor its softness to the touch, nor anything else about it perceptible to the senses. The hat itself (the “substance”) has the shape, the colour, the size, the softness and the other appearances, but is distinct from them. While the appearances, which are referred to by the philosophical term accidents, are perceptible to the senses, the substance is not.
When at his Last Supper Jesus said: “This is my body”, what he held in his hands had all the appearances of bread: these “accidents” remained unchanged. However, the Roman Catholic Church believes that, in accordance with what Jesus said, the underlying reality was changed: the “substance” of the bread was converted to that of his body. In other words, it actually was his body, while all the appearances open to the senses or to scientific investigation were still those of bread, exactly as before. The Church holds that the same change of the substance of the bread and of the wine occurs at the consecration of the Eucharist.
Not bonkers; deeply held religious belief.