Here is a brief excerpt from the exegesis of the text provided by Murray J. Harris in The Second Epistle to the Corinthians (New International Greek Testament Commentary: Eerdmans, 2005):
Originally posted by foudroyant
ὁ δὲ κύριος τὸ πνεῦμά ἐστιν· οὗ δὲ τὸ πνεῦμα κυρίου, ἐλευθερία. "Now this 'Lord' is the Spirit , and where the Spirit of the Lord is, there is freedom." Few sentences in the New Testament have prompted more debate than this linguistically simple statement. Yet there is virtually a scholarly consensus regarding four related issues.
Verse 17a must be interpreted in light of verse 16; both passages refer to a
....κύριος, presumably one and the same κύριος.
Word order and the use of κύριος in verse 16 make it probable that ὁ κύριος
....not τὸ πνεῦμα, is the subject of verse 17a
Because good sense may be made of verse 17b as it stands, there is no need
....to resort to textual emendation.
Because plausible connections of thought can be discerned between verses
....17-18 and verses 7-16, there is no reason to classify verses 17 and 18b as a
....Gnostic marginal gloss incorporated into the text.
[snip 8 pages of exegesis]
If our interpretation of verse 17a is correct ― "Now this 'Lord' (= Yahweh in Exodus 34:34) is in the present era experienced as the Holy Spirit" ― verse 18c may be seen as an abbreviated restatement of this: "the Lord (= Yahweh), who is (now experienced as) the Spirit." On this view, verses 16-18 are pneumatological in emphasis. The new era is the era of the Spirit, for as a result of conversion to the Spirit (verses 16, 17a), there is liberation through the Spirit (verse 17b), including the lifting of the veil of spiritual ignorance and hardheartedness, and also transformation by the Spirit (verse 18). The Spirit, his person and his work, is the hallmark of the new covenant.