It’s an ongoing sin.
Christ said to go and sin no more... not just go back to your adulterous 2nd marriage.
In a case of true hardness of heart where no first marriage truly existed, get an annulment.
Sir, you do not understand Christ’s death on the Cross. What was that for,Sir? When Christ addressed the status prior to His Atoning Sacrifice , the status of God and Man was totally different than after His atonement — right? Or did He die for nothing?