He loves him as the Devil loves holy water, i.e. hates him mortally. Holy water, according to the Roman Catholics, having the virtue to chase away the Devil and his imps.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Holy Lamb * HollowNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes