Of a common Christianity there has never been and never can be a definition, for it has never existed.

 You will find people saying on every side that the Bolshevist movement (for instance) is "definitely anti-Christian and opposed to every form of Christianity" and must be "resisted by all who call themselves Christians.

Speech and writing of this kind are futile because they mean nothing definite. There is no such thing as a religion called "Christianity" there never has been such a religion.

There is and always have been The Church, and various heresies proceeding from a rejection of some of the Church's doctrines by men who still desire to retain the rest of her teaching and morals. 

But there never has been and never can be or will be a general Christian religion professed by men who all accept some central important doctrines, while agreeing to differ about others.

There has always been, from the beginning, and will always be, The Church, and sundry heresies either doomed to decay, or, like Mohammedanism, to grow into a separate religion.

Of a common Christianity there has never been and never can be a definition, for it has never existed.