https://www.desiringgod.org/messages/new-god-new-gospel-new-gladness/excerpts/what-makes-a-christian

Here are the words (transcript) of John Piper’s opening to his sermon at the Together For the Gospel (T4G link above) conference, and it is true: the real acid test of what is a real Christian, is the affections of the heart for Jesus Christ. If you don’t prove with your life that you love Jesus, then you are not a Christian, no matter how much you go to church, give, have been baptized, or even read or own bibles. Real Christians love and follow Jesus because they cannot resist the overwhelming desire to love and follow Jesus. That IS! the work of the indwelling Holy Spirit. Here’s Piper’s words:

“The most basic, most essential distinction between a Christian and a non-Christian is not new decisions of the will, not new deeds of the hands, not new doctrines in the mind, but a new delight in the heart. That’s my thesis. I’ll say it again. The most basic distinction between the body of Christ and the world is not godly decisions, not good deeds, not genuine doctrines, but glad delights in the glory of God, the beauty of God, the excellence of God, the holy majesty of God, the marvelous mercies of God — the person of God as revealed in Jesus Christ supremely.

The world is perfectly able to use its willpower to make decisions for Jesus. Judas certainly did for three years, all the while being a lover of money and a thief, while willing to follow Jesus all day every day. Secular philanthropists do good deeds. “If I give away all I have, and if I deliver up my body to be burned, but have not love, I gain nothing” (1 Corinthians 13:3).

The devil himself knows more right doctrine to be true than anyone in this room. But neither the devil nor secular philanthropists nor the whole unbelieving world does or can delight in the glory of God supremely.”