This week, a member wrote to ask whether 2 Cor 3:17 clearly teaches the deity of the Holy Spirit, or if the original Greek text can be translated other ways. The passage reads, "Now the Lord is the Spirit" (ESV). Her question arose from conversations with Mormons and Jehovah's witnesses who accuse Christians of mistranslating the verse.
It might be more than she bargained for, but here is the text of 2 Cor 3:17:
ὁ δὲ κύριος τὸ πνεῦμά ἐστιν·
Roughly pronounced: "ha deh KOO-ri-os tah NOO-ma ES-teen"
Now, I'm going to break this down word by word:
- δὲ ("deh") = conjunction, "now, however, but"
- ὁ ("ha") = masculine demonstrative article. "the, this, that"
- κύριος ("KOO-ri-os") = "Lord"
- πνεῦμά ("NOO-ma") = "Spirit" (grammatically neuter)
- ἐστιν ("ES-teen") = Present-tense verb of being or equivalence. Basically like our English word "is"
- τὸ ("tah") = neuter demonstrative article "the, this, that"
The first thing to note is that because the neuter article (τὸ) occurs as second in a series of demonstrative pronouns with “the" (ὁ) Lord,” it functions much like our English phrase, “that thing is this.” Moreover, the “Lord” and “Spirit” are joined by the simple verb of being (ἐστιν, “is”). This arrangement, again like English, serves to signify the equivalence of two things.
For these reasons, it seems perfectly clear that the author intended to identify the Lord God as one and the same with his Spirit. “Now, the Lord is that/the Spirit.” In other words, the Holy Spirit is one in being and essence with the Lord. The Belgic Confession, Art. 11, puts it this way:
We believe and confess also that the Holy Spirit from eternity proceeds from the Father and the Son. He is neither made, created, nor begotten, but He can only be said to proceed from both.  In order He is the third Person of the Holy Trinity, of one and the same essence, majesty, and glory with the Father and the Son, true and eternal God, as the Holy Scriptures teach us. 
1. John 14:15-26; John 15:26; Rom 8:9.
2. Gen 1:2; Mat 28:19; Acts 5:3-4; 1 Cor 2:10; 1 Cor 3:16; 1 Cor 6:11; 1 John 5:7.
There is an underlying problem which the questioner might run into, however. Mormons and Jehovah's Witnesses typically base their disputes not on how Christians interpret the Bible, but on whether the biblical texts themselves have been preserved accurately. Their real beef is with God's providential preservation of his Word.
When Mormons come to my door, I try to ask something like, “given that you believe God allowed the Word to be fatally corrupted and lost until the time of Joseph Smith, how do you know he won't do that again?” It is not something they have usually given thought to. This provides an opportunity to express our hope in God's kindness. For although the Bible is sometimes misunderstood, we believe God preserved the Scriptures throughout time just as he promised. “The grass withers and the flowers fall, but the word of our God stands forever” (Isaiah 40:8).
Thanks for asking!
PS: Here's an article from Ligonier on the deity of the Spirit.