In my part of the world, we call this sheep stealing. I frankly detest the practice by Protestants and many Protestants don’t even believe that Catholics are Christians when Catholics are the original Christians! Catholics need to be doing a lot more to counteract Protestant evangelisation attempts.
I usually don’t get upset about sheep stealing because, quite frankly, I wouldn’t hesitate to help any Protestant into the Catholic Church.
However, as a rule, the negative preaching and teaching, the name calling, the condemnations and so forth come from, at least in our time, the Protestant side. To be fair, it comes from a minority within the Fundmentalist segment. Many of the rest, at least among Evangelicals, believe the same thing in their hearts, but are much too charitable to go into attack mode.
God bless them, sometimes I wish we had their evangelical commitment, instead of nuancing the message to death.
IMHO we need to ask ourselves why missionaries to Catholics are successful. In order to do that we need to scope the other side and hear the testimonies of “ex-Catholics” and, if we can get past the rhetoric, get to the heart of the matter. There are also Catholics who have been over to the other side and have come back. Marcus Grodi has had a few guests on his Journey Home program with that experience. Their stories are very instructive as to why they left in the first place. What did they find that they were looking for?
Karl Keating, in his book Catholicism and Fundamentalism, ascribes it to apologetics and doctrine. Of course, he’s in the apologetics business so I would expect him to focus on that part of it, and it certainly is a large part. But I find it hard to believe that someone raised with the Real Presence, Confession, Catholic Tradition and Scripture, and so on, just one day out of the blue reads the Bible and says, “That’s all wrong.”
They accept the doctrines of Sola Fide and Sola Scriptura because they trust the people that teach them. That involves a lot more than apologetics. It often has to do with some grudge they have against someone or something in the Catholic Church. These are the things we have to understand and pay attention to. And, of course, as many have pointed out here, people will not remain faithful to what they do not know or understand. And that is the fault of Catechesis, either at home or in the school or in the Church, or some combination of the three.