Philip Schaff (1819-1893) was a Swiss-born, German-educated Protestant theologian and Church historian, who was a professor at Union Theological Seminary. His 8-volume History is a classic; this fourth volume was originally published in 1866, and last revised in 1889. The next volume in the series is History of the Christian Church, Volume 5, The Middle Ages (Part 1) A.D. 1049-1294 (Volume 5).
Schaff's view of Islam is quite negative. He asserts that "the moving power of Islam was fanaticism and brute force... (Islam) never made an impression on Europe except by brute force... and when it must withdraw from Europe it will leave no trace behind." (Pg. 150) He suggests that Islam was "a well-deserved divine punishment for the unfruitful speculations, bitter contentions, empty ceremonialism... which degraded and disgraced the East after the fifth century." (Pg. 152)
He admits that "The tenth century is the darkest of the dark ages, a century of ignorance and superstition, anarchy and crime in church and state." (Pg. 280) The papacy "became the prey of avarice, violence, and intrigue, a veritable synagogue of Satan. It was dragged through the quagmire of the darkest crimes... Pope followed pope in rapid succession, and most of them ended their career in deposition, prison, and murder." (Pg. 283)
He notes solemnly that the Spanish Inquisition "committed such fearful excesses that even popes protested against the abuse of power... The Inquisition carried the system of torture to its utmost limits. After the Reformation it was still employed in trials of sorcery or witchcraft until the revolution of opinion in the eighteenth century swept it out of existence." (Pg. 352)
He laments, "The single word 'filioque' keeps the oldest, largest, and most nearly related churches divided since the ninth century, and still forbids a reunion." (Pg. 476) He asserts that the doctrine of transubstantiation "reflects a magical supernaturalism which puts the severest tax upon the intellect..." (Pg. 569)
Although there are certainly more recent histories of Christianity, Schaff's has justifiably remained in print since its original publication in the 19th century. It deserves a place on any serious theological bookshelf.