Piper teaches that some Calvinists might not be Born Again.

You are asking about Psa 51:5 I am sure.

Psa 51:5 Behold, I was shapen in iniquity; and in sin did my mother conceive me.

Well, if you read the whole Psalm and do not pull this one verse out of context it is easy to see that is is hyperbole, an exaggeration. Look at some of the other verses in this Psalm.

Psa 51:7 Purge me with hyssop, and I shall be clean: wash me, and I shall be whiter than snow.

Can hyssop purge sin? Can a person really be whiter than snow?

Psa 51:8 Make me to hear joy and gladness; that the bones which thou hast broken may rejoice.

Did God actually break David's bones?

The Psalms are poetry and often use hyperbole or extreme exaggeration. Writers often use this technique, even we do in everyday speech like saying, "Where have you been, I've been waiting here for ages?"

Statements like this are not intended to be taken literally. As I showed with Psa 58:3, children are not born being able to speak, they are not poisonous like an adder, they do not have teeth like a lion. You cannot pull out vs. 3 and interpret it literally and then ignore all the other verses in this same Psalm which are obvious exaggerations.

Look at some of the other exaggerations in Psalms 58:

Psa 58:8 As a snail which melteth, let every one of them pass away: like the untimely birth of a woman, that they may not see the sun.

Do these wicked children really melt like a snail when you pour salt on it? And how can they be born wicked if they are born stillborn?

Psa 58:9 Before your pots can feel the thorns, he shall take them away as with a whirlwind, both living, and in his wrath.

Does God really kill wicked babies with a whirlwind?

So, you should never use scripture that is obviously using hyperbole and exaggeration to form doctrine.

 

In defense of the doctrine of original sin http://www.mountainretreatorg.net/classics/jonathanedwards/original_sin.shtml

But winman if none of us are born sinful then what is the difference between us and Jesus Christ, any sinless man could then have died for the sins of the world because the law required an unblemished sacrifice. is that true?