Totally agree with this. Sabbath keeping included. Observing Sabbath can never improve or add to what Christ accomplished on calvary. So when someone says they observe Sabbath, why is it you do an about face and tell them they are working their way to heaven when here you correctly say such a thing is impossible?
I'm saying that when the Law was in effect it could bring temporal blessings to Israel when they obeyed God's commandments and rules. But that Covenant had a time stamp on it, namely when Israel completely failed under the Covenant to keep their part of the agreement. God had to cancel and annul it.
At the same time, God replaced that failed Covenant with a brand new covenant, the New Covenant. The Old Covenant, wanting to bring blessings to a whole nation, showed that this blessing could not last for any length of time. The Law itself would reveal all of the moral failings within the nation. Even if the failure started small, it spread like leaven all through the lump. It spread like mold all through the house.
So the New Covenant, although it also wishes to bring blessings to godly nations, focuses on final redemption for those who wish to succeed with God, and not turn to apostasy and false religion. If we live in righteousness, we will experience a degree of blessing, together with persecution. If an entire nation turns to righteousness, there will be blessings in that nation.
However, turning to righteousness in the NT time period no longer means turning to rituals of atonement, using physical elements to portray the need for final redemption.
Animal sacrifices only did this to show that Israel needed God's forgiveness regularly, and in the end, needed Christ himself as a perfectly righteous sacrifice. He simultaneously became the source of our forgiveness, by bearing our sins, and the source of our righteousness through the Spirit. And so, we turn to a perfect standard, and to a complete forgiveness.
Sabbath observance is one of those rituals that merely portrayed a need for final redemption. The Sabbath was a regular performance required under that Covenant to display the fact Israel was still unclean by Law, and still prohibited from entering God's eternal Kingdom.
They had to rest periodically, to show the need to end, at least for a time, a display of the sin that prevented them from experiencing final redemption. They had to give the land rest from their regular display of sin, which regularly cursed the land with bloodshed and lawlessness. If they ceased form working, at least for a while, their sins would be quieted, if only externally.
Now that Christ has already provided that final redemption, we no longer need to imitate the need to rest from our sins. They are forgiven. The only thing we have to display is the righteousness of Christ, which was never subject to the Law, since he was sinless. The Law was only given on behalf of sinners.