''Be not afraid!'' counsels the back cover, but actually, the competition has plenty to fear. In its second week out, Crossing the Threshold of Hope by Pope John Paul II has itself crossed the threshold, displacing Faye D. Resnick's sleaze-read on Nicole Brown Simpson to take the No. 1 spot on the nonfiction best-seller list. The 1.25 million copies in print are going fast, and one can't help but suspect divine intervention. Of course, the simultaneously released Spanish-language and large-print editions have helped too.