GHWB cannot be a Knight of Malta because he isn’t Roman Catholic. You must be thinking of a different order.
[GHWB cannot be a Knight of Malta]
Bush Is Knighted in Queen Elizabeth’s Court
December 01, 1993| From Associated Press
LONDON — Former President George Bush, a Texas Yankee in Queen Elizabeth’s court, was knighted at Buckingham Palace on Tuesday.
Bush joined a select group of American presidents, all Republicans, who have been knighted. Because he’s an American, he didn’t have to kneel like an English knight. For the same reason, he is not entitled to call himself “Sir.”
Like Ronald Reagan, Bush was made a Knight Grand Cross of the Most Honorable Order of the Bath—or GCB—the highest honorary rank Britain can give a foreigner.
Dwight D. Eisenhower already had his knighthood when he moved into the White House in 1952. Eisenhower was honored during World War II when he was running the Allied campaign against Adolf Hitler.
http://articles.latimes.com/1993-12-01/news/mn-62855_1_queen-elizabeth-s-court