By Necessity the New Birth Brings Transformation

Much is at stake in seeing the new birth in true biblical proportions. Heaven and hell are at stake—and a church in the world now that acts more like Jesus and less like the culture around it.
Which brings us back to where we started, namely, the claim that born again Christians have lifestyles of worldliness and sin that are indistinguishable from the unregenerate. I don’t think so. 1 John 5:4: “Everyone who has been born of God overcomes the world. And this is the victory that has overcome the world— our faith.” But my conviction is not rosy news for the church. It implies that there are millions of church attenders who are not born again.
Nevertheless, in spite of this conviction, I will distance myself from perfectionism. In other words, I don’t think that the new birth makes us perfect in this life. Sin remains, and the fight of faith is a daily necessity. Some unbelievers look like better people than some believers. But that is because some pretty bad people have been born again, and the process of transformation is not always as fast as we would like.
It’s also because there are unregenerate people who for all kinds of genetic and social reasons conform to an outward morality while being God-indifferent or God-hostile on the inside. God sees the line between the regenerate and the unregenerate perfectly. We don’t. But there is such a line, and those who have been born again are being changed, even if slowly, from one degree of humility and love to the next.
This matters. It matters for eternity, and it matters for the glory of Christ in this life. If people are to enter finally into the kingdom of God (John 3:3), and if the church is to let her light shine on earth that people may give glory to God (Matt. 5:16), then the new birth must be experienced. -John Piper, Finally Alive, p.21.