Islam was once very open-minded towards Orthodoxy, and the Orthodox even feared the Latin Church more then Islam. Muslims during the Ottoman Empire allowed Orthodoxy to survive in a sort of "spiritual oasis" until more recent times, since the Christians, Zoroastrians and Jews, at that time, were defended by the Ku'ran principle to respect "the Peoples of the Book". With the recent events of the last century (I think the key event is the Restoration of Israel), Muslims have begun their fight with the Judeo-Christian/Western World as they (=their leaders) conceive it.
Secularism, and the consequent atheism/agnosticism, is also a great problem. In fact, while Islam works stronglier in the Middle East (but I know of converts both from Europe and North America), secularism is going to mix itself with the historical churches of the Western word. There are lots of clergymen who refuse to believe in Satan and angels as myths or allegories, a fruit of a rebirth of sadducean attitudes. Even Preterists, who deny the resurrection with their idea that the Apocalypse has already been fulfilled in 70 AD, are a result of secularism, of a weak faith in the contents of the Christian religion.
I think that the greater threat to Christianity is Freemasonry. This permeates, in a form or another, every religion one after the other, and is at the base of false ecumenism. Satan could rule all churches and religions easily if all Christians, Jews and Moslems would say "Afterall, WE'RE ALL GOD'S PEOPLE" (sorry for my free reference to a song of the Queen). Reducing all religions to "free opinions" is the best way to strengthen the principal instrument of the Devil, relativism, which is the sceptre of Freemasonry. The recent changes in the Roman Church (including the post-Vatican II "liturgical decay", Communion on the hands and the kiss of pope Wojtila on the Ku'ran), in the Protestant/Anglican Churches (the introduction of women priests) and even in Orthodoxy (with a special reference to the annulment of the excommunications with Rome by Patriarch Athenagoras of Constantinople) are just instruments of Freemasonry, understood more as a "tendency" to relativism whose obscure purpose is to give the world as a present to the Devil. The Church should find more cohesion within itself and resist the temptations towards ecumenism and modern relativism. A good step might be the final reunion with the Copts, Armenians, Ethiopians and Eritreans, whose schism is a fruit of misunderstandings on both sides. A reunion would benefit our missionary activity in the world, especially in those areas now afflicted by Islam (such as the Middle East) and allow the other Orthodox to concentrate on the liberal relativistic West. Of course this is an utopia, but how can't this happen one day, when Jesus himself prayed for Church unity some 2000 years ago?
In Christ, Alex