Marx refered to Christianity as a version of feudal socialism, because it often supported the rich and the powerful aristocrats being in power. That refers to the Christianity of today. Christianity used to a simple religion; Jesus' teachings primarily centered around having a simple, personal relationship with God, rather than relying on superstitious prayer rituals or a church. The Christianity of today is a form that has been corrupted by the power it gained in the Middle Ages.
Jesus' Christianity, I would say had socialist tendencies, but was not totally socialist.
As far as the modern Christianity, I will quote from the Communist Manifesto:
"In political practice, therefore, they [Feudal Socialists] join in all corrective measures against the working class; and in ordinary life, despite their high-falutin phrases, they stoop to pick up the golden apples dropped from the tree of industry, and to barter truth, love, and honour, for traffic in wool, beetroot-sugar, and potato spirits.
As the parson has ever gone hand in hand with the landlord, so has clerical socialism with feudal socialism.
Nothing is easier than to give Christian asceticism a socialist tinge. Has not Christianity declaimed against private property, against marriage, against the state? Has it not preached in the place of these, charity and poverty, celibacy and mortification of the flesh, monastic life and Mother Church? Christian socialism is but the holy water with which the priest consecrates the heart-burnings of the aristocrat."