Wikipedia has reached the maturity to be useful in many cases, though quite some religious pages have been enthusiastically filled by believers and scholars have not yet reached it. This is far from a reason to immediately stop reading. At a lot of passages all that's lacking is the quotation marks and the explanation of why this is taught, which often is clear once you've seen the light. The exploring unbeliever should train to sense when it is a believer who has the word. One has to learn to choose the right Wikipedia entry terms: starters tend to go for far too general terms "Christianity", "religion", ... , or worse, buy a three volume book "The History of Christianity", hoping for an "introduction". It never works. Better be very specific, as children are: ("Mam, what is ... ") ... baptizing, altar, a messiah, a sinner, inquisition ... thus children show us how to efficiently make your own introduction bottom up.

Some useful search terms: Alexandria (history of), altar, Antioch, apocalypse, Apollo, Arianism, Arianus, Aristotle, asceticism, Athanasius, Bagdad (history of), baptizing, basilicas, bishop, Celsus, Classical Antiquity, Constantinople, Copt, Cordoba (history of), Council of Chalcedon, Council of Ephesus, Council of Nicaea, crusade, daemon demon devil Satan evil, emanation, Galileo, god, Goth, heaven, hell, initiation, inquisition, jihad, Julian the Apostate, Jupiter, messiah, miracle, mission, missionary, Moabites, Monotheism, Myth, Napoleon and the papacy, Neo-Platonism, Nestorian, Oracle, pantheism, patriarch, Plato, Platonism, Porphyry, pray, priest, prophet, revelation, ritual, Shia, soul, Sunni, syncretism, temples, Thomas Aquinas, transcendent, trinity, Ulfilas.

Next reading suggestions (free on internet)

The PHiLES pages on Abrahamic religions
Spinoza Theological-Political Treatise (1677), or a summary as in the first chapters of Ratner (1929)
Thomas Taylor. Arguments Of Celsus, Porphyry, And The Emperor Julian, Against The Christians / Also Extracts from Diodorus Siculus, Josephus, and Tacitus, / Relating to the Jews, Together with an Appendix

Hard copy, commercial ebook:

Karlheinz Deschner, Kriminalgeschichte des Christentums (Criminal History of Christianity), 10 Volumes, Rowohlt: Reinbek bei Hamburg, 1986-2013