Europe is developed on Christianity, it could not be imposed upon her. There is no even point to discuss it. First who used it was Alkuin, Charlemagne's religious and educational advisor, in order to dente Western Christians (Frankish empire and its client states, Northern Iberians and Bristish Isles), latter definiton was applied to all western Christians, then to Eastern also. In XVIII century it got modern meaning. Except Albania, all European countries are traditionaly Christian.
Well, concerning slaves, Christianity is relgion of God's children:
Behold what manner of love the Father hath bestowed upon us, that we should be called the sons of God! Therefore the world knoweth us not, because it knew Him not.
Beloved, now we are the sons of God, and it doth not yet appear what we shall be; but we know that, when He shall appear, we shall be like Him; for we shall see Him as He is.