Does anybody here know were classical composers like Bach, Beethoven, Wagner, Haydn or Mozart christians or Antichristians?
You're going to get some really misinformed answers, because you're asking a categorical question and not a syncretic one.
The real question is: what interpretation of religious thought did these composers have?
The answer is that most, while putatively Deists or Christians, embraced a holistic transcendental idealism that enabled them to conceive of the world as whole. This view, best articulated in European thought by Emerson, Blake, Schopenhauer and Eckhart, is what people of an IQ above 145-149 generally reveal as their belief system when asked the right questions.
Similarly, it's impossible to find this out if you ask whether they were believers or atheists. It's not a middle ground -- it's a different direction. The Abrahamic faith has us all looking for an anthropomorphic god in the sky, while the transcendental idealist beliefs of ancient Pagans, Hindus and smart people worldwide have us looking for god in the inherent mathematical patterns of reality.
You are correct about the sharpest of the sharp recognizing that a given religious doctrine points to a holistically united, singular reality... This is something that it is important to understand about religious doctrine generally speaking. The symbolism it employs is not referring to "literal" physical realities, but metaphysics in general.
However, painting the "Abrahamic" faiths as silly cults that worship man-gods beyond the clouds seems intellectually dishonest, or at the very least, ignorant of basic Judeo-Christian concepts of what "God" is, especially within Judaism and Islam. Much more so than the Indo-European religious doctrines as embodied by the Celtic and Nordic beliefs, the Judeo-Christian concept of the divine dispenses with anthropomorphism... In fact, as "pagan" thinker Alain de Benoist points out in a polemic against Judeo-Christianity, the Indo-European concept of the deity really refers to something in a very different category, very much a human-like intelligence or being,
while the "Abrahamic" concept of God refers to something that is much more abstract and less isolated to a specific intelligence.... The Judeo-Christian "god" is the totality/unity of metaphysical principles, as opposed to something that is a manifested, individuated being
. Christianity manages to bridge these distinctions by making Christ the localized divine being
, which is the temporal manifestation of the faceless/formless/abstract, beyond-human absolute.
Anyway, let's not pretend that it hasn't ever been thought by foolish moderns that a belief in the deity Wotan implies a literal belief in a one-eyed, huntsman-sorcerer in the sky. Acting as though all pagans everywhere have always recognized that their pantheonic systems represent principles and metaphysical relationships, while all Judeo-Christians have worshipped a bearded sky-tyrant is incredibly reductive/false.
I think that at some point, ANUS/Corrupt must overcome its confusion about Judeo-Christianity.
To briefly address the original poster - I highly suggest you recognize the incompleteness of your understanding of Christianity and 'religion' in general, before you try to understand its relationship to art. DeathMetalBlackMetal has given you a good starting hint by pointing out that most of great thinkers of Christendom were those who recognized the transcendent unity of reality expressed in their religious doctrines.