Christendom

Christendom is an era when Christianity was first formed. It was the belief of Jesus Christ a Jewish Artisan who taught about the Holy Trinity or the Father, Son, and the Holy Spirit as being one God. Over time Christianity allowed its believers to eat meat and drink alcohol, unlike Buddhism and Islam. In this regard, Christendom was cosmopolitan and transcontinental. 

Christianity also gave people hope for a better life through God and promised an after life. Christendom gained most of it's popularity when the Emperor Constantine of Rome embraced the religion. Since Rome had such a large reach, it was easy for the religion to spread.  

Christendom was gaining popularity as the religion of Islam began to grow in Africa and Asia. The civilizations there quickly embraced Islam causing Christendom to contract in its popularity in those regions. Being a Christian in an Islam state was difficult. In fact, Christians were forced to pay a tax so that they would not be persecuted. 

Christianity continues to be popular today because of its cross-generational belief system and values. Many of the teachings 1500 years ago can be related to today's modern societies. 

References: 

Robert W. Strayer, Eric W. Nelson - Ways of the World_ A Brief Global History with Sources, Combined Volume (2015, Bedford_St. Martin’s) - libgen.lc.pdf

https://goodshepherdcampus.org/understanding-the-holy-trinity/http://www.ibnulyemenarabic.com/arabic-culture/what-is-islam/