It's easy to oppose communism as a Christian when you learn what they did to Christians in Soviet Russia, China, NK etc.
Protestant were persecuted by Catholics as well. All forms of Christians have been violent and engaging in wars against "the others", i.e. anyone who is different minded. One reason America has a freedom of religion was based on certain Christian denominations were persecuted in Europe, and they didn't want the same to happen here.
Besides, Communism persecute all religions, not just Christians. With that said though, Communism wants to create equality for all, give health care to everyone, and create a system that takes care of those in need, but it doesn't work because of human greed. The love of money, power, and influence is forever what corrupts any form of government. But that aside, Communism is an ideology that tries to create the Christian ideals of loving your neighbor (but of course, you can't dictate such things).
Communism, on the other hand, isn't a Christian concept. When they disciples wanted to pay the tax, they didn't have the money, but got it through a miracle. Jesus told them and educated them that God will provide when they have need. He didn't teach them to collect, keep, take, or build wealth at all. Apparently it's easier for a camel to get involved in knitting than a wealthy, selfish Christian to get into heaven.
Jesus might tell them, "I don't know you." and "What you did to the smallest, you've done to me." And right now, the big whale Christians love to screw the small fish over. In fact, some non-Christian wealthy have shown more care for society and those in need. Go figure. Love of money is dangerous, even for Christians.