An "order" usually has more than one member (i.e. a whole group), unless you interpret the "order" to be only typological. The emphasis in the Book of Hebrews is that Christ is of the order of Melchisedec.
However, there are indications in the New Testament that the believers join with Christ in certain things, though believers do not "die for the sins of the world", believers could indeed be of the order of Melchisedec, "All things that the Father hath are mine: therefore said I, that he shall take of mine, and shall shew it unto you." (John 16:15). "Therefore let no man glory in men. For all things are yours" (1 Cor. 3:21). This is connected to the truth that He, "hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever. Amen." (Rev. 1:6).
We find that believers, as priests, are joined to Christ, ordained by Him, and are therefore serving under the order of Melchisedec as typified by Christ, "By him therefore let us offer the sacrifice of praise to God continually, that is, the fruit of our lips giving thanks to his name." (Hebrews 13:15). Note that the believers are now doing priesthood service, but that it is spiritual, that is, truly fulfilling the law, not by carnal ordinances, but by faith and subsequent works.
Many misuse the statement that “we are no longer under the law” to mean that we do not have to have faith, obey the Word, submit to God or by the Spirit do good. Not being under the law means that we are actually able to keep the law by a living and spiritual way: Christ in us.
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