In 2 Corinthians 3 the Apostle Paul is juxtaposing the old and new covenant. He argues that the old covenant, that is represented by letter, death, and condemnation is being superseded by the new covenant represented by the Spirit, life, and righteousness. The old covenant is fading away. The picture presented is Moses from Exodus 34 where Moses comes down from Sinai with the glory of the Lord glowing on his face. The brightness led to the placing of a veil over the head of Moses.
According to the Apostle the veil had another function: to prevent the sons of Israel from realizing the glory was fading. He seems to be depicting Moses as fearful of the possibility that the fading glory would cause the Israelites to lose their awe of him. This principle is transfered to those who currently are still in awe of Moses (read Torah) who do not realize that the glory of Moses (the old covenant) is fading because there is a symbolic veil prevented this realization.
Those who have the Spirit are the ones who not only can see past the veil and therefore can truly understand Moses, but also are like Moses in Ex. 34.34 who removes the veil when he turns to speak to YHWH. Like Moses, the veil is “removed” from those who are Spirit-filled when they turn to see the Spirit.
There has been some who has read this passage in confusion. Since Christ is referred to in v. 14 these people argue for a modalistic reading of this passage by seeing Christ as the antecedent to “the Lord” in vv. 17-18. This reading would equate Jesus with the Spirit. To understand the misreading, in order to present a correct reading, we should examine the passage. I have provided vv. 12-18 below (from the NASB):
12 Therefore having such a hope, we use great boldness in our speech,13 and are not like Moses, who used to put a veil over his face so that the sons of Israel would not look intently at the end of what was fading away.14 But their minds were hardened; for until this very day at the reading of the old covenant the same veil remains unlifted, because it is removed in Christ.
17 Now the Lord is the Spirit, and where the Spirit of the Lord is, there is liberty.18 But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as from the Lord, the Spirit.
As you can see it is possible to misread this text by suggesting that the Apostle sees “the Lord” as referring back to Christ in v. 14. The error of this reading is that Christ in v. 14 is not the antecedent of “the Lord”. The Lord, as previously alluded to above, is God in Ex. 34.34a. This part of the verse reads:
But whenever Moses went in before the Lord to speak with Him, he would take the veil off until he came out;
In v. 16 “whenever a person turns to the Lord the veil is taken away” has Ex. 34.34 in view. Therefore, in v. 17 when the Apostle writes, “the Lord is the Spirit” he is not thinking of Jesus in v. 14 but rather God in Ex. 34.34. Gordon Fee puts it this way,
By “the Lord,” Paul does not intend either God or Christ; he intends the Spirit. That is, he is interpreting the text of Exodus in light of the present argument. The Lord in that text, he is saying, is now to be understood (not literally, but in an analogical way) as referring to the Spirit–not because this is the proper identification of the Lord in the Exodus text, but because in this argument that is the proper way to understand what happens to those who, as Moses, now “turn to the Lord.” The Spirit, who applies the work of Christ to the life of the believer, is the key to the eschatological experience of God’s presence. With the veil removed from the hardened heart, God’s people enter into freedom. 
Fee sees the Spirit as functioning in the same way as “the Lord” of Exodus 34 in that when someone/Moses turns to the Spirit/the Lord the veil (literal and symbolic) is removed. Once the veil is removed there is clarity. Only the Holy Spirit does this. 
So for those who understand the statement “the Lord is the Spirit” to be a modalistic affirmation that Jesus is the Holy Spirit the serious problem with this reading is that it ignores that the antecedent to the Spirit is found in Ex. 34.34, not v. 14.
 Gordon D. Fee, God’s Empowering Presence: The Holy Spirit in the Letters of Paul, 311-312.
 I disagree with Fee that this is necessarily “analogical”. I do not see why this passage cannot mean that “the Lord” of Exodus 34.34 is the Holy Spirit. Nevertheless, Fee’s reading is a possibility but not a necessity.