In a previous post on Matt26:26/Mark 14:22/Luke 22:19/1 Cor 11:24 and the phrase, “this is my body” (τοῦτό ἐστιν τὸ σῶμά μου in Matt 26:26) I argued that the term “is” (ειμι) is an interpretive ειμι, and cited some instances of this construction in the New Testament undercutting the Roman Catholic (and to a lesser degree, Eastern Orthodox and Lutheran) understanding of this phrase and its theological significance towards a theology of the Eucharist. As further evidence, I offer the following segment under the entry ειμι in Bauer, Danker, Arndt, and Gingrich (BDAG), A Greek-English Lexicon of the New Testament and Other Early Christian Literature (3d ed.):
c. in explanations:
α. to show how someth. is to be understood is a representation of, is the equivalent of; εἰμί here, too, serves as copula; we usually translate mean, so in the formula τοῦτ᾽ ἔστιν this or that means, that is to say (Epict., Ench. 33, 10; Arrian, Tact. 29, 3; SIG 880, 50; PFlor 157, 4; PSI 298, 9; PMert 91, 9; Jos., C. Ap. 2, 16; ApcMos 19; Just., D. 56, 23; 78, 3 al.) Mk 7:2; Ac 19:4; Ro 7:18; 9:8; 10:6, 8; Phlm 12; Hb 7:5 al.; in the sense that is (when translated) (Polyaenus 8, 14, 1 Μάξιμος ἀνηγορεύθη· τοῦτο δ᾽ ἄν εἴη Μέγιστον) Mt 27:46; Ac 1:19. So also w. relative pron.: ὅ ἐστιν Mk 3:17; 7:11, 34; Hb 7:2. After verbs of asking, recognizing, knowing and not knowing (Antiphanes Com. 231, 1f τὸ ζῆν τί ἐστι;) μάθετε τί ἐστιν learn what (this) means Mt 9:13. εἰ ἐγνώκειτε τί ἐστιν 12:7; cp. Mk 1:27; 9:10; Lk 20:17; J 16:17f; Eph 4:9. W. an indir. question (Stephan. Byz. s.v. Ἀγύλλα: τὶς ἠρώτα τί ἂν εἴη τὸ ὄνομα) τί ἂν εἴη ταῦτα Lk 15:26; τί εἴη τοῦτο 18:36. τίνα θέλει ταῦτα εἶναι what this means Ac 17:20; cp. 2:12, where the question is not about the mng. of terms but the significance of what is happening.—Esp. in interpr. of the parables (Artem. 1, 51 p. 48, 26 ἄρουρα οὐδὲν ἄλλο ἐστὶν ἢ γυνή=field means nothing else than woman) ὁ ἀγρός ἐστιν ὁ κόσμος the field means the world Mt 13:38; cp. vss. 19f, 22f; Mk 4:15f, 18, 20; Lk 8:11ff (cp. Gen 41:26f; Ezk 37:11; Ath. 22, 4 [Stoic interpr. of myths]). On τοῦτό ἐστιν τὸ σῶμά μου Mt 26:26; Mk 14:22; Lk 22:19 and its various interpretations, see lit. s.v. εὐχαριστία. Cp. Hipponax (VI BC) 45 Diehl αὕτη γάρ ἐστι συμφορή=this means misfortune.
For a related, more technical argument about the grammatical structure of Matt 26:28 and the claim Jesus taught that His blood was shed at the Last Supper, see my analysis and refutation, "Was Christ's Blood Shed at the Last Supper?"