11 And [kai] he received [lambano] the sign [semeion] of
circumcision [peritome], a seal [sphragis] of the righteousness [dikaiosune] of the faith [pistis] which
[ho] he had yet being [en] uncircumcised [akrobustia]:
that [eis] he [autos] might be [einai] the father
[pater] of all [pas] them that believe [pisteuo],
though [dia] they be not circumcised [akrobustia]; that [eis] righteousness [dikaiosune] might be imputed [logizomai]
unto them [autos] also [kai]: KJV-Interlinear
received the sign of circumcision as a seal of the righteousness that he had by
faith while he was still uncircumcised. The purpose was to make him the father
of all who believe without being circumcised, so that righteousness would be
counted to them as well, ESV
A seal is a legal mark that ratifies and makes binding, an agreement between two or more parties. Typically wax was used and then a mark of some sort was imprinted in the wax.
The ritual of circumcision, is a symbol of the seal of the promise from God to Abraham.
Today we have notaries who validate the signatures, and the contract they sign is binding when recorded on some public record for all to see and acknowledge.
Back in Abrahams day, God made the circumcision, which is an irreversible mark on the male, as the symbolic seal, ratifying the agreement between God and Abraham, which agreement was extended to all of humanity. Gods promise dates back to Adam, and before that, the divine decrees which were written in eternity past.
Later, after the Cross, the seal was changed to a more binding seal, which is the indwelling of the Holy Spirit, which comes to everyone who believes in Christ, and it occurs at the moment of their initial belief. The seal is binding and irreversible.
And thus, Abraham became the father of all believers, by means of this pattern of faith.
All believers believe in order to receive salvation.
Once saved, always saved.
Faith is irreversible.
Gods fulfilment of His promise is irreversible.
Birth is irreversible and the rebirth is likewise irreversible.
You cannot unborn yourself.
You cannot undo your initial act of faith. Once done, it is done.