One common response to John Piper's thoughts on baptism and church membership is to say that being unbaptized is to live in sin, and anything besides immersion as a believer is not baptism, therefore those who have not been immersed after their profession of faith are, in fact, living in sin. In Mark Dever's recent post on the issue, he writes,

Jesus clearly commanded baptism and to disobey this command is sin [whether intentional or not]. To continue in such an unbaptized state is unrepentant sin [whether intentional or not].

His point being that if we as Baptists do not believe infant baptism is legitimate, then we can't in good conscience have paedobaptists in our membership or at our communion table. However, Dever also believes that these "unrepentant sinners" can be our brothers in Christ (even though they are not permitted in our fellowship). He points out that he is going to have both an Anglican and a Presbyterian in his "Baptist pulpit" in the next few months.

It seems that Dever has a category for an unrepentant Christian. But what about 1 John 3:8?

Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning.

The words "unrepentant sin" can come across as relatively benign, but the Apostle John wants to make perfectly clear what they mean: if you are unrepentantly sinful, you are "of the devil." And there is no such thing as a Christian who is of the devil.

What does this mean for those who are wrong about baptism?

It means just that—they're wrong. But being wrong and being an unrepentant sinner are not the same. If they were, everybody with an imperfect theology (all of us) would be lost. But instead of going to hell, a believer can come before God with humility and repentance and say, "I'm weak-minded and fallible. I'm sorry that I do not understand you like I should. Please help me to know you more." And after this, he may still believe in infant baptism. Not because he's unrepentant, but because he's fallible.

John Piper writes,

On almost every disagreement that Christians have about what should be believed and what should be done, someone is living in "unrepentant sin”—that is, they are not doing what they ought to do and they are not confessing it as sin.... The question is: what do we do with all these Christians who can't agree on what ought to be done and therefore do the wrong things and don’t confess it as sin? I argue that every Christian has always had a category of behaviors and a category of attitudes about behaviors that they believe are not what God wants us to do, but do not consign a person who does not believe the same to the category of church discipline. We agree to disagree in the faith, because we agree that the other person is earnestly seeking for the light of God and that the nature of the sin is not such that it calls their faith into question or threatens their soul or undermines the gospel. Admittedly this is muddy, but it seems inevitable (Phil. 3:15)." (from an email)

Jesus looks at mankind and sees two groups: the sheep and the goats; the saved and the damned; the repentant and the unrepentant (Matthew 25:31-46). Unless we are willing to say that paedobaptists are probably going to hell, then they are—according to Christ—already members in our fellowship, his church. What's left for us is to acknowledge this. And then to trust Jesus to show himself to his one bride.