Meyer's NT Commentary
http://biblehub.com/commentaries/meyer/2_corinthians/3.htm
2 Corinthians 3:17. Remark giving information regarding what is asserted in 2 Corinthians 3:16.
δέ, [the German] aber, appends not something of contrast, i.e. to Moses, who is the letter (Hofmann), but a clause elucidating what was just said, περιαιρ. τὸ κάλ.,[175] equivalent to namely. See Hermann, ad Viger. p. 845; Hartung, Partikell. I. p. 167. Rückert (comp. de Wette) is of a different opinion, holding that there is here a continued chain of reasoning, so that Paul in 2 Corinthians 3:1617 means to say: “When the people of Israel shall have turned to the Lord, then will the κάλυμμα be taken from it; and when this shall have happened, it will also attain the freedom (from the yoke of the law) which is at present wanting to it.” But, because in that case the ἘΛΕΥΘΕΡΊΑ would be a more important point than the taking away of the veil, 2 Corinthians 3:18 must have referred back not to the latter, but to the former. Seeing, however, that 2 Corinthians 3:18 refers back to the taking away of the veil, it is clear that 2 Corinthians 3:17 is only an accessory sentence, which is intended to remove every doubt regarding the ΠΕΡΙΑΙΡΕῖΤΑΙ ΤῸ ΚΆΛΥΜΜΑ.[176] Besides, if Rückert were right, Paul would have continued his discourse illogically; the logical continuation would have been, 2 Corinthians 3:17 : ΟὟ ΔῈ ΠΕΡΙΑΙΡΕῖΤΑΙ ΤῸ ΚΆΛΥΜΜΑ, ΤῸ ΠΝΕῦΜΑ ΚΥΡΊΟΥ ἘΣΤΊΝ· ΟὟ ΔῈ ΤῸ ΠΝ. ΚΥΡ. Κ.Τ.Λ.
Ὁ ΔῈ ΚΎΡΙΟς ΤῸ ΠΝΕῦΜΑ ἘΣΤΙΝ] Ὁ ΚΎΡΙΟς is subject, not (as Chrysostom, Theodoret, Oecumenius, Theophylact, Estius, Schulz held, partly in the interest of opposition to Arianism) predicate, which would be possible in itself, but cannot be from the connection with 2 Corinthians 3:16.[177] The words, however, cannot mean: Dominus significat Spiritum (Wetstein), because previously the conversion to Christ, to the actual personal Christ, was spoken of; they can only mean: the Lord, however, is the Spirit, i.e. the Lord, however, to whom the heart is converted (note the article) is not different from the (Holy) Spirit, who is received, namely, in conversion, and (see what follows) is the divine lifepower that makes free. That this was meant not of hypostatical identity, but according to the dynamical oeconomic point of view, that the fellowship of Christ, into which we enter through conversion, is the fellowship of the Holy Spirit, was obvious of itself to the believing consciousness of the readers, and is also put beyond doubt by the following τὸ πνεῦμα κυρίου. And Christ is the Spirit in so far as at conversion, and generally in the whole arrangements of salvation, He communicates Himself in the Holy Spirit, and this Spirit is His Spirit, the living principle of the influence and indwelling of Christ,—certainly the living ground of life in the church, and the spirit of its life (Hofmann), but as such just the Holy Spirit, in whom the Lord reveals Himself as present and savingly active. The same thought is contained in Romans 8:911, as is clear especially from 2 Corinthians 3:1011, where Χριστός and ΤῸ ΠΝΕῦΜΑ ΤΟῦ ἘΓΕΊΡΑΝΤΟς ἸΗΣΟῦΝ and ΠΝΕῦΜΑ ΧΡΙΣΤΟῦ (2 Corinthians 3:9) appear to be identical as the indwelling principle of the Christian being and life, so that there must necessarily lie at the bottom of it the idea: ΧΡΙΣΤῸς ΤῸ ΠΝΕῦΜΑ ἘΣΤΙ. Comp. Galatians 2:20; Galatians 4:6, Php 1:19, Acts 20:28, along with Ephesians 4:11. As respects His immanence, therefore, in His people, Christ is the Spirit. Comp. also Krummel, l.c. p. 97, who rightly remarks that, if Christ calls Himself the light, the way, the truth, etc., all this is included in the proposition: “the Lord is the Spirit.” Fritzsche, Dissert. I. p. 42, takes it: Dominus est ita Sp. St. perfusus, ut totus quasi τὸ πνεῦμα sit. So also Rückert, who nevertheless (following Erasmus and Beza) believes it necessary to explain the article before πνεῦμα by retrospective reference to 2 Corinthians 3:6; 2 Corinthians 3:8.[178] But in that case the whole expression would be reduced to a mere quasi, with which the further inference οὗ δὲ τὸ πνεῦμα κυρίου would not be logically in accord; besides, according to analogy of Scripture elsewhere, it cannot be said of the exalted Christ (and yet it is He that is meant), “Spiritu sancto perfusus est,” or “Spiritu gaudet divino,” an expression which can only belong to Christ in His earthly state (Luke 1:35; Mark 1:10; Acts 1:2; Acts 10:38); whereas the glorified Christ is the sender of the Spirit, the possessor and disposer (comp. also Revelation 3:1; Revelation 4:5; Revelation 5:6), and therewith Lord of the Spirit, 2 Corinthians 3:18. The weakened interpretation: “Christ, however, imparts the Spirit” (Piscator, L. Cappellus, Scultetus, and others, including Emmerling and Fiatt), is at variance with the words, and is not to be supported by passages like John 14:6, since in these the predicates are not concretes but abstracts. In keeping with the view and the expression in the present passage are those Johannine passages in which Christ promises the communication of the Spirit to the disciples as His own return (John 14:18, al.). Others have departed from the simple sense of the words “Christ is the Spirit,” either by importing into τὸ πνεῦμα another meaning than that of the Holy Spirit, or by not taking ὁ κύριος to signify the personal Christ. The former course is inadmissible, partly on account of the following οὗ δὲ τὸ πνεῦμα κυρίον, partly because the absolute τὸ πνεῦμα admits of no other meaning whatever than the habitual one; the latter is made impossible by 2 Corinthians 3:16. Among those adhering to the former view are Morus: “Quum Dominum dico, intelligo illam divinitus datam religionis scientiam;” Erasmus and Calvin: “that τὸ πνεῦμα is the spirit of the law, which only becomes viva et vivifica, si a Christo inspiretur, whereby the spirit comes to the body;” also Olshausen: “the Lord now is just the Spirit, of which there was mention above” (2 Corinthians 3:6); by this is to be understood the spiritual institute, the economy of the Spirit; Christ, namely, fills His church with Himself; hence it is itself Christ. Comp. Ewald, according to whom Christ is designated, in contrast to the letter and compulsion of law, as the Spirit absolutely (just as God is, John 4:24). Similarly Neander. To this class belongs also the interpretation of Baur, which, in spite of the article in τὸ πνεῖμα, amounts to this, that Christ in His substantial existence is spirit, i.e. an immaterial substance composed of light;[179] comp. his neut. Theol. p. 18 7 f. See, on the contrary, Räbiger, Christol. Paul. p. 36 f.; Krummel, l.c. p. 79 ff. Among the adherents of the second mode of interpretation are Vorstius, Mosheim, Bolten: “ὁ κύριος is the doctrine of Jesus;” also Billroth, who recognises as its meaning: “in the kingdom of the Lord the Spirit rules; the essence of Christianity is the Spirit of the Lord, which He confers on His own.” For many other erroneous interpretations (among which is included that of Estius, Calovius, and others, who refer ὁ κύριος to God, and so explain the words of the divinity of the Holy Spirit), see Pole and Wol.