Moreover, the Christianity of which we speak is that which we find realized in the Catholic Church alone; hence, we are not concerned here with those forms which are embodied in the various non-Catholic Christian sects, whether schismatical or heretical.

I. ORIGIN OF CHRISTIANITY AND ITS RELATION WITH OTHER RELIGIONS

Christianity is the name given to that definite system of religious belief and practice which was taught by Jesus Christ in the country of Palestine, during the reign of the Roman Emperor, Tiberius, and was promulgated, after its Founder's death, for the acceptance of the whole world, by certain chosen men among His followers.

If the selection were made by Christ and His adherents, rationalists must tell us how these "ignorant and unlettered men" (Acts 4:13; cf. Matthew 13:54; Mark 6:2) knew the religions of the East, when it was a matter of astonishment to their contemporaries that they knew their own.

II. THE ESSENTIALS OF CHRISTIANITY
(1) The Teaching of Christ
(2) The Teaching of the Apostles

III. THE DIVINE PURPOSE IN CHRISTIANITY
(1) Universality includes both space and time
(2) Christianity is meant to be a perfect religion
(3) God intended, in the third place, that Christianity should be a visible organization.

I don't feel like reading it, but it's here...
http://www.newadvent.org/cathen/03712a.htm