Verse 18 says that not a jot or tittle of the Law will pass until heaven and earth have passed away.
... or until all is accomplished (fulfilled).
Christians will point to Jesus' cry of "It is finished!" on the cross as an indication that all has been accomplished in regard to our salvation.
Where in the Hebrew scriptures does it indicate that someone else can fulfil the Law on your behalf?
... as an atoning sacrifice ...Leviticus 16:29 And this shall be a statute for ever unto you: that in the seventh month, on the tenth day of the month, ye shall afflict your souls, and do no work at all, whether it be one of your own country, or a stranger that sojourneth among you:
30 For on that day shall the priest make an atonement for you, to cleanse you, that ye may be clean from all your sins before the LORD.
So heaven and earth have passed away?
... as an atoning sacrifice ...
Precisely what form of sacrifice was Jesus?