This is from a letter written to Lucius Clay, who at the time (1947) was the military governor for Germany. He tried to resign his post (officially) at least eleven times; unofficially, the count was even higher. Eisenhower had been his good friend for a long time, and in this letter, he was trying to talk him out of one of the threatened resignations. I found it quoted in The Candy Bombers: The Untold Story of the Berlin Airlift and America's Finest Hour, by Andrei Cherny.
"...please remember that now abide Faith, Hope, and Charity, these three, and greater than any is a sense of humor."