I. Not everyone who claims to be a Christian is. Drunks on the street say they believe.
Unmarried couples sleeping together say they believe.
Elderly people who haven't sought worship or fellowship for 40 years say they believe. All kinds of lukewarm, world-loving church attenders say they believe. The world abounds with millions of unconverted people who say they believe in Jesus. Piper is describing unbelieving believers. Some profess to believe, but do not.