From the National Catholic Register's Matthew Warner:
"According to a 2009 Pew Forum on Religion and Public Life, four people leave the Catholic Church for every one person that joins it. Keep in mind that this stat doesn’t count those born into Catholicism as “joining” it. However, it’s still a sad statistic. But we shouldn’t be misled by it.
There are also studies that show Catholicism has a higher rate of retention than all other religious groups. In other words, when people convert to Catholicism, they don’t do so because they didn’t like where they were and just wanted to try something new. Their conversion is deliberate and intentional and they generally stick with it. On the other hand, when people leave the Church, they generally drift around a bit from one denomination to another. This says a lot. The Catholic convert is actually experiencing real, lasting conversion. Those leaving the Church seem to be lost and searching souls that most likely had no idea what they were leaving in the first place.
I’ve long noticed, as have many others, a kind of trend as well. It’s not so much from “Evangelicals” converting to Catholicism necessarily. It’s that of intellectuals converting to Catholicism. And that’s not to say these intellectuals were strictly intellectual. But I mean it to say that they took their reasons for believing very seriously. We only have to look back a few generations to find Chesterton, Merton, Newman, etc. as part of the same trend.
In my own experience, I’ve seen that more people who convert to Catholicism do so on account of their reason. Whereas those that leave the Church do so based on some emotion or negative experience associated with the Church.
When I ask an evangelical why they left the Church. The answer is almost always an emotion. Something made them feel a certain way. Or they just didn’t like the way something was done in Catholicism. Or it didn’t suit their lifestyle. Or some other experience made them feel nice..."