There is materialism, and there is materialism. In common usage, materialism is the tendency to consider material possessions and physical comfort as more important than spiritual values. In philosophy, materialism is the doctrine that nothing exists except matter. Materialism is already well ensconced in the academic world and, because of that, quickly becoming the philosophy of the world in which we live.
The truth is that we do not live in a merely material world. We live in a world formed, maintained, constrained, and shaped by God. We don't live in a world where gravity, as an example, is the primary force. We understand that if you place a ball on a ramp, it will roll down. Always. We get that. We tend, then, to think of all of life like that. We Christians tend to think of all of life like that. Here are some examples ... taken from Christians. "If Hillary (or Trump, depending on your bent) gets elected, our country will descend into worse conditions than ever before." "If we don't fight for our religious freedoms, we will lose them and God's work in the world will suffer." Something I've actually heard from more than one Christian: "The way things are going I can't imagine that the Church -- Christianity itself -- will survive." This is Christian materialism. It is making decisions and taking positions based on what we see, measure, and understand, not on God.
I heard the story of a man who spent his life teaching seminary. When he retired, he sold all he had and moved to a Muslim country to take them the gospel. Most of the world would say that was nuts. Many Christians would say it wasn't wise. And when we learn that he was killed there, it is simply proof. Bad move. This is Christian materialism, measuring life by what we see rather than what God is doing.
We do it, every one of us, all the time. Some of us worry about where the next check is coming from because the world around us says that you need a job to get by and we just got laid off. This doesn't stump God. He isn't caught off guard. It is our materialism showing. Parents have largely become "helicopter parents", even among Christians, because it's a dangerous world out there and we need to watch our kids every minute. This assumes no God or, at best, a God severely limited by His creatures. "My son is on drugs" or "My daughter is leaving her husband" or "What are we going to do about this election year?" are all cries of genuine anguish, but are not informed by a world that is maintained, constrained, and shaped by God. They are Christian materialism, the trials that arise because all we know and all we can trust is what we can see, measure, and understand in human terms.
The truth is, dear Christian, we don't live in a merely material world. We live in a world held together by God (Col 1:17). He does what He pleases (Psa 115:3; Psa 134:6). Power and might are in His hand so that no one can stand against Him (2 Chron 20:6). To limit ourselves to the world we live in seems a given from a materialist perspective. But we are, in truth, actually strangers rather than citizens (1 Peter 2:11). Our Christian materialism, however, makes no sense in an existence where God is on the throne.