Matt 6:18 κρυπτω . . . κρυπτω

Very few witnesses (ℵ B [D] f1 pc) twice have κρυφαιω instead of κρυπτω, as in most (including E G K L M S U V W Γ Δ Θ Π Σ Φ Ω 0233 0250 [Byz 1500+ mss] f13 33. 565. 892. 1424). Griesbach's explanation (Commentarius, 1:73–4) is classic: "Κρυφαιω should not at all be spurned. For this form of the word is not used in the New Testament, but it occurs several times among the Greek translators of the Old Testament. The Westerns, less meticulous in these things, preserved the rarer form; the Alexandrians along with the Byzantines substituted the more familiar [form], which in this very chapter had preceeded four times in vss. 4 and 6 and, having implanted itself in the memory of scribes, began to adhere to this place." Von Soden (1:1014) similarly explains, "As the difference from 6:4, 6 was hard to understand for a scribe, who was much more inclined toward assimilation, so the agreement of ℵ-B with documents completely unrelated to this archetype necessitates the admission of κρυφαιω into a respected class, thus in H [=Egyptian] or I [Jerusalem]."
     The reason for hesitation in accepting the rarer κρυφαιω is primarily methodological but also based on the corresponding internal consideration that the κρυπτ- form is demonstrably Matthean (5:14; 6:4 [twice], 6 [twice]; 10:26; 11:25; 13:33, 35, 44 [twice]; 25:18, 25; Mark's sole instance is in 4:22), whereas the κρυφ- form is otherwise unattested in Matthew (but does occur in Mark, also in 4:22). To quote Hort (2:34–5 [§43]):

[T]he difference between isolated judgements and combined judgements is vital. In the one case any misapprehension of the immediate evidence . . . tells in full force upon the solitary process by which one reading is selected from the rest for adoption, and there is no room for rectification. In the other case the selection is suggested by the result of a large generalisation about the documents . . . [and] rests on too broad a foundation of provisional judgements, at once confirming and correcting each other, to be materially weakened by the chance or probability that some few of them are individually unsound."

     Robinson and Pierpont state the same principle in the preface to their edition (viii): "Byzantine-priority theory does not operate on an eclectic variant-by-variant basis. Rather, it continually investigates the position of all variant units within the history of transmission" (emphasis mine). It is this principle that prevents scholars, e.g., from receiving the rarer φαγεσθε in 1 Cor 10:27, even though it is the earliest attested reading (cf. p46) and appears in distinction to the otherwised harmonized εσθιετε, which appears in 10:25 and 28.
     Consequently, the reason for κρυφαιω's rise in a few witnesses of Matthew, just as for p46's φαγεσθε in 1 Cor 10:27, is necessarily speculative. Matthäi (91) remarks, "Perhaps κρυφαιω was obtained from Origen, whose commentary unfortunately is not extant at this place." Fritzsche (274), on the other hand, greatly wonders whether κρυπτω might not have been "rashly refined into κρυφαιω by those who had referred to God as 'the hidden God' [occulto Deo] from a wrong interpretation of κρυπτω." And in fact Eusebius (Demonstratio evangelica 5.4.4–15 [GCS 9/3:224–6]) abandons the LXX rendering of מסתתר in Is 45:15 as και ουκ ηδειμεν ("and we did not know [him]") in favor of the translations of Aquila (αποκρυπτομενος), Theodotion (κρυφαιος), and Symmachus (κρυφαιος), saying that "very wonderfully does [Isaiah] call Christ the 'Hidden God,'" and using this text to argue for the eternal existence and godhead of Jesus. But neither do Eusebius' interpretation nor Western theology's important Deus Absconditus concept (which is more about absence than blessing) fit very well internally at this place in Matthew.
     On the other hand, the introduction of the synonym κρυφαιω may have been simply an early editorial preference to escape the repition of κρυπτω (4 times in the previous few verses), or for some other unknown reason. In this regard it is not insignificant that the Alexandrian Cyril (Commentarii in Lucam 22.40 [PG 72:921A]), when citing the model prayer in Matthew 6:6, says "προσευξαι τω Πατρι σου τω εν τω κρυφαιω· και ο Πατηρ σου ο βλεπων εν τω κρυφαιω αποδωσει σοι εν τω φανερω," even though no known manuscripts have κρυφαιω in Matt 6:6 (all have κρυπτω there).
     Finally, we cannot agree that the mere alignment of D and f1 with the archetype of ℵ-B constitutes an outstanding class of evidence. A casual glance at the many coincidences of those witnesses in the first six chapters of Matthew proves that they are not so "completely unrelated" as von Soden claims. Further, it is the combined judgment of the continual sequence of the readings of manuscripts in the manuscript tradition, and not individual and arbitrary judgments without regard to such, that causes the consensus text's κρυπτω to be preferred; that term is also demonstrably more Matthean, while the other is not.