Are Christians idealists, in the sense in which a Marxist would use the term? You know I am at university in a Marxist country.
I think you may be asking any of four different things.
Possibility one: Is a human being at bottom just a body (a material substance), or is he at bottom just a soul (an intellectual substance)? We believe that human beings are not bodies alone, or souls alone, but souls united with bodies. Many analytical philosophers call this view “dualism,” although I dislike the term because it can have more than one meaning. The more traditional name for the doctrine of body-soul unity is “hylomorphism.”
Possibility two: Do nonmaterial forms really exist, or is matter all that exists? Certainly forms exist. Matter exists in time, but time is not matter. We have thoughts, but thought is not matter. A sentence has a meaning, but even though the meaning may be represented by ink marks on paper, the meaning is not the same thing as the ink marks on paper; in fact, the meaning is the same even if the matter changes – for example, if the sentence is represented by configurations of pixels on a computer screen.
Possibility three: Granted that these forms exist, do they exist in themselves, as Plato thought? The traditional Christian view is that they don’t, so if that is what is meant by idealism, we are not idealists. Rather forms are found in three other places -- in things, in minds, and in the mind of God – though they are not found in these three places in the same sense, but in analogical senses. The form of the body “informs” the body; my grasp of this form is in my mind; and all forms of created things preexist in the mind of the Creator. This view is usually called moderate realism – realist, because it affirms the reality of forms, and moderate, because it does not suppose that they have independent existence.
Possibility four: A Marxist would probably be thinking not just of Platonic idealism but of Hegelian idealism. I am no Hegel scholar, but subject to correction, I understand him to have thought that the world cannot be distinguished from how the world is understood, and that God or “Spirit” becomes conscious of itself in human minds. Orthodox Christianity rejects these views. God, who brought our minds and the world into existence, is distinct from our minds and the world. Simultaneously whole in every moment, He has no need to evolve. Knowing all things, He has no need of us to be conscious of Himself. Our minds are true when they correspond to how things are; things are as they are because they correspond to the mind of the Creator.