Christendom did not exist, prior to the 4th-century.
It is- essentially- a European religion that is gentile in doctrine and teaching.
Christianity- which did not permit gentiles to become disciples, until they had discarded their gentile religious philosophies- was thoroughly Judaic in heritage and teaching.
Christians were not bound by the Mosaic or Oral Laws, but were well educated in their principles, and the Hebrew Scriptures, which are frequently quoted in the Greek Scriptures. Gentile concepts, like the immortal soul, were not accepted in the Christian Congregation, who taught that humans were mortal, died, and awaited a resurrection in the "new earth". (described in the books of Isaiah, 2 Peter and Revelations.)
Important differences between Christianity and Christendom:
Christianity: Non-political, Non-commercial, Non-nationalistic, Focus is on teaching and offering the "good news", Worships Jehovah as God and "heavenly father", Relies on God's power- or "holy spirit", Views Jesus as a "mediator" between man and God.
Christendom: Highly-political, Highly-commercialized, Nationalistic, Labors to 'Christianize' nations, and mobilize populations, Worships God as Trinity, Views God's "holy spirit" as an unnamed person, within a Trinity 'Godhead', Worships Jesus as God.