Must Christians obey the Old Testament?

[True Federalist (진정한 연방 주의자)]

There is one aspect of OT law that most Christian churches blatantly ignore that is still in force, the Sabbath.  Granted, they have a day of rest one day per week, but it is the wrong day.

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Note that it is possible to worship on Sunday and keep the Sabbath on its proper day if one wishes.  Nowhere in the Bible is worship ever restricted to just the Sabbath day.

[True Federalist (진정한 연방 주의자)]

So you are saying God lied about the seventh day being perpetually ordained as the sabbath?

I could see the argument that the perpetual covenant to follow the seventh-day sabbath laid out by God in Exodus 31:12-17 applies not to the Gentiles, but only to the Jews, as an obligation of their status as his "kingdom of priests" (Exodus 19:6), but holding such a distinction between Jew and Gentile would be at odds with the structural supersessionism you espouse.

[True Federalist (진정한 연방 주의자)]

If you accept the fact that both the NT and OT are of God then there is no contradiction there. This is a matter of presuppositions. If you don't accept the authority of the NT you're going to see contradictions, and if you do accept it then you won't.

What contradictions?  I don't see any verse of the NT that explicitly sets aside the Saturday sabbath, and even those who torture the text to find an implicit setting aside can find no authority in the NT for establishing a Sunday sabbath in its place.  (Unless you make the mistake of conflating the Sabbath with the Lord's Day.)

There are fairly few places in the OT where YHWH speaks of something as being perpetual. The seventh day sabbath is perpetual. The Aharonic priesthood is perpetual, the covenant to never flood the earth again is perpetual.  Perpetuity is not something that God takes lightly in the OT. When he says perpetual, he damn well means perpetual.