"Now the LORD spoke to Moses, saying, “Speak to the children of Israel, saying: ‘If a person sins unintentionally against any of the commandments of the LORD in anything which ought not to be done, and does any of them, if the anointed priest sins, bringing guilt on the people, then let him offer to the LORD for his sin which he has sinned a young bull without blemish as a sin offering..."
Those who insist on keeping all 613 mitzvot make the blood worthless ("For it is impossible for those who...fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.", as Hebrews 6 partly reads.). They like to play pick-and-choose mitzvot while under grace, since they cannot possibly go to the Temple and offer a korban l'kippur.
Besides, "the law, having a shadow of the good things to come, and not the very image of the things, can never with these same sacrifices, which they offer continually year by year, make those who approach perfect. For then would they not have ceased to be offered? For the worshipers, once purified, would have had no more consciousness of sins. But in those sacrifices there is a reminder of sins every year. For it is not possible that the blood of bulls and goats could take away sins.", as Hebrews 10 partly reads.
To advocate that keeping all 613 mitzvot while under grace is a mitzvah, is a sin. Legalism is sin, and "if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. Anyone who has rejected Moses’ law dies without mercy on the testimony of two or three witnesses. Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace?"
Those who insist on keeping all 613 mitzvot make the blood worthless ("For it is impossible for those who...fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.", as Hebrews 6 partly reads.). They like to play pick-and-choose mitzvot while under grace, since they cannot possibly go to the Temple and offer a korban l'kippur.
Besides, "the law, having a shadow of the good things to come, and not the very image of the things, can never with these same sacrifices, which they offer continually year by year, make those who approach perfect. For then would they not have ceased to be offered? For the worshipers, once purified, would have had no more consciousness of sins. But in those sacrifices there is a reminder of sins every year. For it is not possible that the blood of bulls and goats could take away sins.", as Hebrews 10 partly reads.
To advocate that keeping all 613 mitzvot while under grace is a mitzvah, is a sin. Legalism is sin, and "if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. Anyone who has rejected Moses’ law dies without mercy on the testimony of two or three witnesses. Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he was sanctified a common thing, and insulted the Spirit of grace?"