Where are we told the new birth is only for sinners?
Jesus Christ is, was and forever will be God's Son.
We on the other hand must be born of God to become children of God.
We are not automatically children of God at our natural birth.
We become children of God by believing on His Son, through the Gospel Message.
Whoever believes that Jesus is the Christ
is born of God, and everyone who loves Him who begot also loves him who is begotten of Him.
1 John 5:1
and again -
But as many as received Him, to them He gave
the right to become children of God, to those who believe in His name:
who were born, not of blood, nor of the will of the flesh, nor of the will of man, but
of God.
John 1:12-13
You must be
born of God to be His child.
having been born again, not of corruptible seed but incorruptible, through the word of God which lives and abides forever,
1 Peter 1:23
A son of God is a being that is created directly or Born of God.
Flesh gives birth to flesh, and The Spirit gives birth to spirit.
A sinner is a reference to someone
who is not a child of God.
A sinner is a reference to a person who is still under sin; an unrighteous person. Not in right standing with God.
7 For if the truth of God has increased through my lie to His glory,
why am I also still judged as a sinner? 8 And why not say, "Let us do evil that good may come"?--as we are slanderously reported and as some affirm that we say. Their condemnation is just. 9 What then? Are we better than they? Not at all. For we have previously charged both Jews and Greeks that they are all under sin. 10 As it is written:
"There is none righteous, no, not one; Romans 3:7-10
JLB