Piper explains why just saying "Believe" is not enough:
"First we are surrounded by unconverted people who think they do believe in Jesus. Drunks on the street say they believe. Unmarried couples sleeping together say they believe. Elderly people who haven't sought worship or fellowship for forty 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.
It does no good to tell these people to believe in the Lord Jesus. The phrase is empty. My responsibility as a preacher of the gospel and a teacher in the church is not to preserve and repeat cherished biblical sentences, but to pierce the heart with biblical truth.