The word “Christendom” refers to Christianity in culture. That is, in areas of the world where Christianity is the state religion, or at least the culturally normal and acceptable form of religion, the practice of Christianity usually suffers grave distortion from the teaching of Jesus. Most students of Christian history date the beginning of Christendom to AD 313 when Constantine promulgated the “Edict of Toleration”. Since then Christianity has been marred by this “Constantinian Error”.
People attend church and hold beliefs for social reasons as well as for spiritual reasons. In Christian cultures there is, therefore, a wide variety of expressions of Christianity being practiced at the same time.
When secular government and church government are in the same hands the abuses and distortions are especially dramatic. This has been true in history, with medieval Europe and early colonial America being prime examples.
When humans corrupt the gospel message of Jesus the usual result is two forms of Christianity being practiced in the same place. The distorted forms and the true forms coexist in the same culture and sometimes in the same denominations, parishes and local churches.
Unfortunately, most people cannot even distinguish the fact that multiple forms of Christianity are being simultaneously pursued around them. For example, in the 19th century, in both England and the United states, some Christians were protecting slavery and some Christians were working for its abolition. Yet today most people will tell you that “Christianity” was a pro-slavery religion since so many Christians were indeed slave holders and vocal supporters of slavery. They cannot distinguish between true Christianity and Christendom.
In order to gain a true knowledge of God though a person must come to understand that distorted, false and corrupt practices of Christianity do not reflect on Jesus. The real question is why God allows evil. Why does He allow Christians to discredit Him? Why does He allow non-Christians to be hindered from coming to Jesus because of false Christians?
We are currently seeing a judgment against the developed western world that is predominantly a judgment against Christendom. This will most likely continue and eventually result in a persecution of Christians that will separate the true believers from the false and purify Christianity.