Lady Gwendoline Bertie's letter to Winston Churchill has the line:
“If you come into
contact with Islam your conversion might be effected with greater ease than
you might have supposed, call of the blood, don’t you know what I mean, do
fight against it.”
The "call of the blood" phrase was puzzling... it seemed like an in-joke. By chance tonight I read that Henry Stanley, third Baron of Alderley, converted to Islam. Now Lord Stanley's sister was Henrietta Blanche, the grandmother of Churchill's wife.
Unfortunately the timing doesn't quite work: Churchill courted Clementine in 1908 and the letter was written the previous year.
Footnote: according to Nancy Mitford, at the third Baron's Islamic funeral, his brother Algernon turned to the new Lord Stanley, who had removed his hat as a mark of respect, and snapped "Not your hat, you fool, your boots".