For truly I say to you, means that what He is about to say, is the truth.
When know Jesus is the truth.
We also know that in this verse contains the words; from the law till all is fulfilled.
By these words from Jesus we know the law of Moses was temporary.
It's easy to see that verses 17 and 18 are firmly connected. Just as verses 18 and 19 are. All three together are talking about the instruction of the law of Moses and the prophets, not just the prophecies about Jesus in the law and the prophets.
The phrase Law and Prophets is a reference to all the Torah which includes Genesis as well as parts of Exodus which were before the Ten Commandments were given at Mount Sinai.
In other words, the passover Lamb was a picture and prophetic type of Jesus Christ, which was fulfilled.
No one is arguing that the righteous requirements that were contained in the Abrahamic Covenant, and were seen in the law of Moses, have been done away with, but the law of Moses with it's commandments and ordinances have been abolished and declared obsolete and have vanished away.
If you believe the commandments within the law of Moses are still for the Church today, under the new covenant, then you had better obey them and show everyone that you believe they are for the church by obeying them.
So that you can start today, the Sabbath, and stone to death everyone who is in violation of that commandment.
JLB