"'as a Gentile and a tax collector'"

Matthew "means by the verb [(Terminus)] 'sins' [in 18:15] not every little misadventure, every mistake, not the venial sins that can creep up on the man and [the] Christian; rather, 'sins' designates those cases in which a Christian [1] seeks the life of another [Christian] or [2] misleads him into an apostasy from the faith.  In this most wicked of cases, the fellow Christian cannot and may not look on in indifference, but must help and protect.  To put it concretely, when Judas goes about his wicked work, Peter is not permitted to remain idle.  The protection of the [1] life and [2] faith of the third [party] is the necessary and sufficient reason for church leadership [(Kirchenleitung)] and church discipline.  Where the perpetrator can be converted off of [(von . . . abbringen)] his wicked path, that is good.  But where not, the community must then tell him what he has himself done [(bewirkt)].  He has, by his own behavior. excluded himself from the community.  The phrase 'as a Gentile and a tax collector' denotes precisely the impossible possibility of being deprived, as a Christian—from even the [merely] human point of view [an] incalculable [loss]—, of the [Christian] community and salvation [itself]. . . .

     ". . . the keyword ἁμαρτάνω in v. 15 is closely linked with another word that occurs in [v. 6]:  σκανδαλίζω:  to sin within the community means to endanger a brother or sister in [2] faith and/or [1] life.  Here there can and may be none of that neutrality that Matthew otherwise so clearly enjoins on his community.  Church discipline becomes necessary when (and only when) a [member] of [(in)] the community stands in need of th[is] protection-and-help-for-the-third-[party].

     "The procedure that Matthew here prescribes for his community carries the procedure for bilateral conflicts known from the 'Testament of Gad' [(Testaments of the Twelve Patriarchs)] and Qumran into the three-way relationship.  This means that the situation addressed by the New Testament text is not covered by the cases just cited [(TestGad 6, 1Q 5:24 ff., CD 9:2 ff)].  The relative mildness of the early Jewish texts cannot be opposed to the sharpness and unmercifulness of Matthew.  On the contrary:  the procedure laid down by Matthew is of a breadth and circumspection that stands in striking opposition to even the later penitential prescriptions of the Church, as Origen, for example, so obviously gives us to understand in his interpretation of th[is very] passage.

     "That the three-stage process of admonition is meant to effect the conversion of the one confronted requires no further proof.  [But] If the worst case of public obliviousness of guilt [(Schuldvergessenheit)] obtains [(if, that is, the sinner, having been 't[old] his fault', 'refuses to listen even to the church')], then the community—obviously the local community—can at that point only bear witness to what the sinner [has] already effected [(praktiziert)]:  Community is by him already so obviously renounced that it can no longer be healed by [any] human power."

     Christoph Kähler, "Kirchenleitung und Kirchenzucht nach Matthäus 18," in Christus bezeugen:  Festschrift für Wolfgang Trilling zum 65. Geburtstag, ed. Karl Kertelge, Traugott Holtz, and Claus-Peter März, Erfurter theologische Studien 59 (Leipzig:  St. Benno-Verlag GMBH, 1989), 140, 144 (136-145).