sourceContextually it is clearly observed that the letter is referring to the Law while the spirit refers to the Spirit.
Whenever the spirit of God is there is liberty,that is Emancipation from bondage,Freedom.The bible made us to understand that if the Spirit that raise Jesus Christ be in us ,he shall quicken our mortal body. 
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The book of John made it so clear that "of His fullness have we receive grace for grace.We believers are no longer under the law but under the grace.
Verse 6:who also hath made us able ministers of the new testament ,not of the letter,but of the spirit,for the letter killeth,but the spirit giveth life.
The law was written by moses in the old testament in time past but thank God whom has made us ministers if the new testament through Christ,we have eternal life and the Holy spirit dwells in us.
Verse 17:Now the Lord is that Spirit,and where the spirit of the Lord is ,there is liberty
Verse 18:But we all with open face beholding as in a glass the glory of the Lord,are changed unto the same image from glory to glory,even as by the Spirit of the Lord.
In Christ we are changed from living in the law for the glory that surpasses it to living in His Spirit. 
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Glory to God!!!
Smartiz