18) Are Christians United?

Christianity is not united. In the course of history the Christian Religion has divided into different denominations. But Christians living on earth are bound together by Jesus' teaching on brotherly unity. The number of Christians is about one billion.

See:  For just as the body is one and has many members and all the members of the body, though many, are one body, so it is with Christ. For in the One Spirit we were all baptized into one body - Jews or Greeks, slaves or free - and we were all made to drink of one Spirit.   (1 Corinthians 12:12-13)

19) Why is Christianity divided into different denominations?

Among the Christians of antiquity and later in the age of the Reformation, different thoughts about the personality and teaching of Jesus arose. Denominations have been formed from the followers of these different religious conceptions.