What he was saying was that the Jews were careful in conforming to the letter, the physical, part of the law, but they were woefully remiss about the spiritual part of the law. Here's another example from scripture:
Luk 11:42 But woe unto you, Pharisees! for ye tithe mint and rue and all manner of herbs, and pass over judgment and the love of God: these ought ye to have done, and not to leave the other undone.
Jesus didn't CONDEMN keeping God's laws (in this case tithing), he said it ought to be done, but he took exception to them not doing the spiritual part of tithing, which is based upon justice and love of God.
If you keep the spirit of God's laws, you DO keep the letter.
Keeping the Sabbath is the important part, doing it on Sunday instead of Saturday contradicts the letter of the law, but not the spirit of the law.
Let's try another commandment. Can a married man physically have sex with another woman but spiritually be keeping the adultery law? Of course not. Your actions follow your heart. You do what your heart says:
Mat 5:27 Ye have heard that it was said by them of old time, Thou shalt not commit adultery:
Mat 5:28 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Christ fulfilled the law. He made law keeping not just a physical act, but a spiritual, mental and physical act. His spirit under the new covenant allows us to keep his righteous requirements.
So what theologian, modern or otherwise, can you name that thought worship on a Sunday was anathema to God?
Off the top of my head DR. SAMUELE BACCHIOCCHI. There are others, but I don't spend a lot of time studying theologians opinions.
The spirit of the law is more important than the letter of the law.
If you base your faith on faithful observance, replicating the means and practices or language of observance in the time of Jesus rather than concentrating on turning your heart towards Jesus, you have put form over function and have built your faith upon a foundation of sand rather than stone.
Christians worship on Sunday, they eat pork, many are or have historically been uncircumcised, they celebrate Christmas with the remnants of Yule celebration and celebrate Easter with remnants of a fertility festival. This is our European heritage, and I don't believe for a second that God has a problem with it, otherwise the Holy Spirit would not have found such fertile soil among our cultures pagan fore-bearers.
Function over form. The spirit not the letter.
But go ahead and tell people to not get Christmas Trees, to not paint eggs for Easter; for me that time would be better spent telling them about the love of Jesus the Christ.