I got a question today about using tefillin, more commonly called (among Gentiles) phylacteries, in worship. Tefillin are the leather bands with small leather boxes which contain scrolls with passages from the Torah (most commonly Deu. 6:4) that Orthodox and Hasidic Jews wear in prayer.
There's nothing wrong with using tefillin in worship. Whlie Yeshua criticized those who made their tefillin over-large to show off their "piety" (Mat. 23:5), He did not condemn the practice itself and even paired it off with the Biblical practice of wearing tzitzit, or fringes.
The tradition comes from Deu. 6:6, 8, in which YHVH commands, "And these words, which I command thee this day, shall be in thine heart . . . And thou shalt bind them for a sign upon thine hand, and they shall be as frontlets between thine eyes." Certainly, wearing tefillin is a very literal way of fulfilling this command. However, as I explained in the post on living a symbolic life, it's not the whole fulfillment.
In Exo. 13:16, YHVH commands Israel to keep the Passover every year, saying, "And it shall be for a token upon thine hand, and for frontlets between thine eyes: for by strength of hand the LORD brought us forth out of Egypt." Therefore, we must interpret Deu. 6 in the light of the previous command in Exo. 13, and conclude that what God is saying that we must do symbollic acts ("bind them for a sign upon thine hand") and view symbollic things ("they shall be as frontlets between thine eyes") to remind us of God's commands and all that He in His boundless grace has done for us.
Literally binding the tefillin to one's hand and head certainly qualifies as keeping this command in both a very literal and yet also a symbolic way. However, it is not the only, or even the primary, means by which we are to keep this commandment, nor should it be considered a requirement. When we keep the Passover, we keep the command. When we celebrate the Lord's Supper, we keep the command. When we put on a talit and look on the tzitzit, we keep the command. When we are baptized, we keep the command. All of these things involve doing and looking upon physical symbols of the spiritual reality that we are a part of, and serve to keep God's Word frontmost in our mind.
Therefore, while a Messianic Jew or former Gentile may use tefillin as a part of his (or her, but that's another subject) worship, he is not required to, nor should he do so simply to show off how holy or Torah-observant he is.