The Prime Minister hinted at his belief in God over the weekend but refused to be drawn on whether he was a practising Catholic.
Boris Johnson was quizzed about his faith by ITV's Robert Peston after marrying Carrie Symonds in Westminster Cathedral last month.
But he fobbed off the question, saying, "I don't discuss these deep issues, certainly not with you."
When Peston pressed the subject further by referring to Labour leader Sir Keir Starmer's atheism, Johnson responded by saying, "The foolish man has said in his heart there is no God," taken from Psalm 14.
Writing on the ITV website after the interview, Peston said he was "struggling to make sense of" the Prime Minister's refusal to answer the question, which he said had been asked "in good faith and with good reason" given his Westminster Cathedral ceremony.
"He is aware that - for better or worse (worse for a long time) - this has been a pertinent question for chief and prime ministers since Henry Vlll," Peston said.
"More broadly, the professed faith or none of a leader matters to many voters."
Peston added, "There is nothing in my 35 years as a journalist to suggest that I would trivialise or denigrate religion, or any issue of conscience."
Johnson is the first baptised Catholic to be in office at No 10. Prior to his Catholic wedding with Carrie, the couple had their infant son, Wilfred, baptised Catholic last September September.
Although former Prime Minister Tony Blair is a practising Catholic, he was an Anglican while in office and only converted after leaving No 10.