George Barna is America's Christian pollster. He has simultaneously supported the growth of the mega church by providing polling data while also writing books extremely skeptical of evangelical practices.
The reason that John Piper levies that charge of blasphemy against George Barna is because Barna has lumped a wide variety of people into the group that may be identified as "born again." Barna's polls have consistently found that there are no differences between those that are identified as born again and the rest of the world. Barna does not create the born again polling group by allowing them to self-identify. There are a number of questions that he asks that, per his definition, dilineates born-againdom.
Piper takes umbrage because he believes that Scripture teaches that the Holy Spirit makes a difference in a person's life once they accept Christ. Per Piper's perspective, a saved individual is by definition a changed individual. Saved people cannot continue to live like the rest of the world because the power of an omnipotent God is working His sanctification in them.
Piper is right. When we tell the world that true Christians are not different from anyone else, we are telling the world a lie. If a person does not act like a redeemed person, they are not a redeemed person. Jesus gave us two standards for identifying Christians; profession of the truth and lifestyle. If either of these are missing, the person is not known by Christ.
It is a form of blasphemy to say that Jesus' work is not efficacious. Since only God can know the human heart, George will probably continue to fight the blasphemy dragon since his definition does not take into account the God prescribed human evaluations of truth and lifestyle.
I'll conclude by letting John Piper speak for himself:
In other words, in this research the term "born again" refers to people who say things. They say, "I have a personal commitment to Jesus Christ. It’s important to me." They say, "I believe that I will go to heaven when I die. I have confessed my sins and accepted Jesus Christ as my Savior." Then the Barna Group takes them at their word, ascribes to them the infinitely important reality of the new birth, and then blasphemes that precious biblical reality by saying that regenerate hearts have no more victory over sin than unregenerate hearts.