To the best in Christendom: i.e. the best **** in Christendom; a health formerly much in vogue.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Bess-1 * BetNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes