Those who are born again have the “witness” in them. In 1 John 5:10, the word translated as “witness” is a Greek word that means “evidence.” So, those who are born again believers have the evidence placed within them. It is not a secret evidence, nor is the evidence a feeling one has, or a declaration one makes. The evidence is just that—evidence. If you have not been given the evidence, then it cannot be your evidence.
Stating that one has been given the Holy Spirit because of his own declaration, his own understandings, or his own performance of a man-made ritual (like “accepting Christ”) is not evidence, it is hearsay. It is a claim for which no actual evidence has been given. And, nothing that has been written can ever tell an individual that he is born again—so, quoting scripture is not the evidence.
The evidence that is given is the confirmation of the Holy Spirit. And, His indwelling is the real substance that physical circumcision foreshadowed. Those who were physically circumcised were physical Jews. Those who are spiritually circumcised (hearts) are spiritual (true) Jews—and, the spiritual is the real substance, the physical is only a foreshadow.
Christianity being a counterfeit of the true faith that was invented by men who stripped the foundations of the faith away, identifying with that belief system is a counterfeit of the identification of true believers. Paul explains that those who have the Holy Spirit are the true Jews (the real substance is spiritual) in Romans 2:29.
Those who are indwelt by the Holy Spirit are Jews both by adoption (justification) and by marriage (covenant). Those who are christians identify with a belief system that was invented to obscure the things that identify those true Jews—they reject the sign given between Yah and His children (His Sabbaths), and they even reject the true names of the Father and the Son. So, one can either be a Jew or a christian, but not both. Those who are Christ’s according to Romans 8:9 are Jews, as He is a Jew. And, that has nothing to do with anything physical.
For more information: Circumcision and Spiritual Reproduction