A former Divinity School student charged with threatening the life of President Pusey was arrested Saturday night by the F.B.I. in Sun Valley, Idaho.

Joseph Steyskal, 38, was arraigned before a U.S. Commissioner in Twin Fall, Ida., for violation of the federal extortion statute by sending Pusey a threatening letter through the mails, the F.B.I. said.

Steyskal, who attended the Divinity School from September 1953 to the following June, formulated his threat on the grounds that the University refused to grant him a professorship to teach religion.

"The man is paranoid," explained Dr. Dana L. Farnsworth, Director of the University Health Services, who filed the complaint against Steyskal in Boston Federal Court on Friday, when Pusey received the letter. Farnsworth said that the F.B.I. assistance was sought to protect Pusey and to get hospital treatment for Steyskal.

Farnsworth revealed that Steyskal, who immigrated from Czechoslovakia in 1946, has been writing to Pusey for about a year and a half in an attempt to get an appointment to teach here. His previous letters have been answered "in as gentle a fashion as possible," Farnsworth said.

Steyskal's psychotic condition, Farnsworth said, led the ex-divinity student to believe he had been "called upon" to preach a new philosophical doctrine. It was only when his illness led him to pick on one individual that Steyskal became "too dangerous to remain at large," Farnsworth stated.