I believe the definition of "Jew" is a loaded one. That is, it is used in several different ways. In the sense of a Torah-observant individual, I believe that this hearkens to resting in observation of times, seasons and Laws - which I believe is seen in Paul's letter to the Galatians and encapsulated here :
But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?
Ye observe days, and months, and times, and years.
I am afraid of you, lest I have bestowed upon you labour in vain.
Galatians 4:9-11
Jesus fulfilled the feasts 2000 years ago (though I know many say His returns fulfills one....at least all but that one are fulfilled). So in that sense, observing the feasts is rejecting the Messiah. Not only that, if the feasts were given to Israel, could it be (and I simply ask...not assert) that one is trying to be a Jew in the sense that Revelation 2:9 and Revelation 3:9 condemn? After all, Paul said that he is not a Jew which is one outwardly, but is one inwardly.
For that reason, I see the church as a continuation of God's covenant with Abraham. Jesus called those that do the works of Abraham (walk by faith) the children of Abraham. So the sense looked for is not in hearkening back, but in recognizing the fulfillment in Christ.
Based on that, I find it hard to believe someone could be observant of Jewish ceremonies without at least being tempted to "go back" into bondage. Not that it can't be done, but that it involves more than just appreciation of one's spiritual heritage.
The Law is a good mirror. It's an awful perfecter. Always has been...always will be. So to observe the Law again goes to something deeper than just not killing, not committing adultery etc... It has to do with what a man is resting on. If one starts resting on the law, they will be doing what the Galatian church did and what Paul came against.
This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
Galatians 5:16-17
Then Paul and Barnabas waxed bold, and said, It was necessary that the word of God should first have been spoken to you: but seeing ye put it from you, and judge yourselves unworthy of everlasting life, lo, we turn to the Gentiles.
Acts 13:46