Well forceful conversion is bad. If people are converting to religions like Christianity and Islam with their own will, why should you curse them. Indeed we all know those idols can not benefit us and we have been deceived by our so called pandits / scholars for long time. You will have good life if you donate a cow, where in Hindu scripture it says that?
I do not know about Christianity but few of my friends have converted to Islam by their own will and they live in United States. I have seen them being a very good person infront of me. They used to drink, smoke and also date girls; now they are married with Muslim girls, they do not drink or smoke. I think they have become better human being. Any religion that makes any human being better is good.
Let us not be ignorant like this mother in this video: https://www.youtube.com/watch?v=11rsLkQdVUY&noredirect=1