Here's the explanation: God is holy and cannot except anything less than His own perfect righteous standards. God is also just and therefore He must punish sin. God cannot just look the other way because then He would be condoning unrighteousness. Sin must be punished by God if He is to be just Himself. Being just Himself God must punish sin and therefore sin has a penalty and that penalty is death and eternal separation from God. Jesus Christ became God in the flesh and He perfectly fulfilled God's law therefore enabling Him to take the penalty for our sin upon Himself on His cross of death. Jesus Christ became the substitute for sinners on the cross taking their penalty upon Himself and therefore satisfying God's righteous standards.