I'll try my best and answer your question.
Matthew 5:17 "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them"
Well what does fulfill mean?
Irenaeus a late 2nd century Christian states that "The Lord did not abrogate the natural of the Law but he extended and fulfilled them"
"Extended?" you may say. "This is unscriptural!" However just looking at the rest of Matthew 5 easily confirms this.
"You have heard that it was said to the people long ago, `Do not murder, and anyone who murders will be subject to judgment.' But I tell you that anyone who is angry with his brother will be subject to judgment"
"You have heard that it was said, `Do not commit adultery.' But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart"
"You have heard that it was said, `Love your neighbor and hate your enemy.'But I tell you: Love your enemies and pray for those who persecute you"
The law has been extended and fulfilled. We are now of the spirit rather than the flesh.
Paul does this also.
Paul appeals to the Law in 1 Corinthians 9:8 but later goes on to say that God doesn't care about Oxen.
Paul also says in Colossians 2:17 that Law of the Jews was a shadow of the things to come.
Jesus does the same in Matthew 7:18-19 by saying that nothing can defile him if it doesn't enter his heart. Jesus hasn't abolished the law but he has extended and fulfilled it.
Hebrews 7:12 says there's a change of priesthood and of the law.
So what has this got to do with the Sabbath?
I can assure that NONE of the early christians kept a day Sabbath. To claim so is either out of naivety or a lie.
Here's a quote from Ingatius of Antioch. Head of the church of antioch and appointed by the position by the Apostle John. This is from one of his letters in the very early 2nd century.
"Those who have been brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath but living in observance of the Lord's day"
The Letter of Diognetus written in the first half of the 2nd century
"But as to their meticulousness concerning foods, their superstition regarding the Sabbaths, their boasting about circumcision.. as if He forbad us to do what is good on the Sabbath days, how is not this impious"
Justin Martyr an incredibly important church theologian and figure writing in the mid 2nd century
"The new law requires you to keep perpetual Sabbath, and you, because you are idle for one day, suppose you are godly, not understanding why this command was given to you"
Irenaeus who was also a very important defender of Orthodoxy writing in the late 2nd century
"And there will be no command to remain idle for one day of rest, to him who perpetually keeps sabbath, that is to say, who in the temple of God, which is man’s body, does service to God, and in every hour works righteousness."
However the early church also kept the Sabbath. Though they no longer did work on the 7th day, they kept the new version of the Sabbath or the perpetual sabbath. Instead of literally resting they now spiritually rested in Christ and gave everyday (which was now holy due to keeping the Sabbath) to the Lord.
Hebrews 4:4 is talking about 7th day Sabbath which contrasts the new Sabbath of Hebrews 4:8-9.
For the record, the Lord's day mentioned by Ignatius is completely different than the Sabbath. You didn't fast on the Lord's day (for reverence of Christ resurrection ) but you still worked.
So now we Christians keep everyday holy and worship God everyday too instead of just one. Resting in Christ everyday rather than in the flesh once a week. Keeping a perpetual Sabbath.
