I don't want to go too deep into this, but I'd like to make a few points of understanding. First off, many in the Messianic movement do what they do for different reasons. Some Jews do Jewish things for cultural reasons, some out of a sense of identity, some out of a sense of duty. Some even out of a sense of obligation. However, most Jews, whether Messianic or not, do not believe Gentiles have ever been required to follow the Law of Moses, now or in times past. The minority of people who do are mostly Gentile Christians who have joined the Messianic movement. And while I won't argue that Gentiles should observe Moses, I also hope that none of you get upset if we do.
In re the verses that Jim quoted, one is interpreted quite differently by many Messianics and the other actually has clues to the contrary of what was suggested.
The Greek word translated "change" (metathesis) by the KJV can also be translated as "move", thus the Greek could be saying...."For the priesthood being moved, there is made of necessity a move also of the law." The Greek version seems to suggest a "change" or "moving" (metathesis) between the earthly priesthood and the heavenly priesthood of Yeshua. Many Christians have trained people to see this as a "change" FROM the Levitical TO the heavenly, but perhaps its really talking about FROM the heavenly TO the earthly. Because the earthly was patterned after the heavenly and the heavenly existed before the earthly one. Yeshua did not take over the earthly Levitical priesthood - it was a mere rehearsal of the heavenly priesthood He had always possessed. In other words, Monte has read this as saying...
"For the priesthood being changed (from Levitical to Melchetzedek) there is made of necessity a change also of the law (From using Levites to Yeshua)." (Heb 7:12, KJV with my comments in commas)
...when the more correct understanding would be...
"For the priesthood being changed/different (from the heavenly priesthood that always existed to the earthly Levitical that came afterwards) there is made of necessity a change/difference also of the law (From using the eternal Son to using earthly men subject to death, inheriting priesthood by birth, but not firstborn status as with Yeshua)." (Heb 7:12, with my comments in commas)
There was more than one CHANGE in the priesthood. Earthly men were allowed to be earthly priests by virtue of firstborn status in Genesis, which was ONE change from teh heavenly order. That was CHANGED again in Numbers 3 from the firstborn to the Levites.
Now if you've been taught all your life to read Heb 7:12 to mean a change from Levitical to Yeshua, that may be a new twist to you. But to interpret that way is very problematic, because it means He was not priest beforehand. Yet I would say He was priest from Eternal past. So no, I do not think that Hebrews 7:12 is some sort of smoking gun that proves we should all abandon doing anything Jewish.
Also, Hebrews 8:13 says this in the King James,
"In that he saith, a new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away." (KJV of Hebrews 8:13)
Keep in mind that this is saying that the "old" has not "vanished away" yet. It says it is "READY", but that it has not done so yet, implying a present tense.
At http://messiahalive.com/passover4cups.htm is an explanation as to why we are presently living in the betrothal stage of the New Covenant. We are not in the marriage stage of it yet and won't be in the marriage stage of the New Covenant until the following verse is fulfilled;
"no longer will a man teach his neighbor 'know the Lord' for they shall all know me....they will all be teach by the Lord". (Jer 31:34)
That does not describe today, thus, the "New Covenant" spoken of in this verse has not yet come. The betrothal stage of it has come, but we still wait for the marriage stage.
When it does come, what will it look like? Ezekiel 40-48, and the end of Zechariah both talk about the temple being rebuilt, about Passover and Sukkot and the weekly Sabbath being celebrated, and sacrifices offered at the temple. That is how the Scriptures describe the final stage of the New Covenant when Yeshua sets up His kingdom on earth.
So if you want nothing to do with anything Jewish, don't get mad at the brother next to you who wants to get a little more prepared for how the Scriptures describe the Millenial Kingdom by doing those things today. If it was a commandment to Israel at Mount Sinai, and it is part of our future, it can't be sin today or something we should label as a "cult". I realize there are some Christians out there who think God realized that the Law was His one big mistake and decided to do away with it, but there really are no killer verses out there that simply disprove that the Messianic philosophy is evil, and no where does the New Testament tell Jews to quit practicing the Law. I do think the New Testament is clear that Gentiles are under no obligation to keep it, so if you don't want to, don't worry about. Just don't get upset at people who want to learn something from it by practicing it.
Shalom,
Joe