This statement taken from II Corinthians 3:6 was recently questioned in a comment from another post. The question is a good one. Because it is a life/death issue (scripturally speaking), let's wrestle with it a bit.
The law or the letter or the letter of the law kills; important principle at work here. Especially law in religion kills. The questioner asks the obvious; are we not to obey God's commandments? Any other answer than yes would be foolish.
So how does this work if we aren't to operate by law so the Spirit can work. First, enter the mystery of God for He isn't lying to us. Second, I remind you I am a mere man trying to awaken us to this mystery via explanation I am incapable of giving. Dig?
The context is law versus liberty (:17). Liberty doesn't fit our heritage well. We've relied upon rules of performance rather than faith of Spirit. Therefore, this discussion is quite difficult for our particular ears to hear. The truth is law kills and Spirit gives life. God said such.
And I was once alive apart from the Law; but when the commandment came, sin became alive, and I died; and this commandment, which was to result in life, proved to result in death for me; for sin, taking opportunity through the commandment, deceived me, and through it killed me (Rom. 7:9-11).
For if a law had been given which was able to impart life, then righteousness would have indeed have been based on law. But the Scripture has shut up all men under sin, that the promise by faith in Jesus Christ might be given to those who believe. But before faith came, we were kept in custody under the law, being shut up to the faith which was later to be revealed. Therefore the Law has become our tutor to lead us to Christ, that we may be justified by faith. But now that faith has come, are no longer under the tutor (Galatians 3:21-25).
Freedom is a Christ element. We are expected to partake of the perfect law of liberty (Jas. 1:25). Yet, we don't because we have viewed our present condition as having moved from one law to a new one; from one tutor to a new one while we tend to miss the freedom and liberty in Jesus. The pull is so strong we feel the need to explain away our freedom for we wouldn't want our people to believe we could just be free to do anything we wanted....like believe in possibility and hope and wonder and awe.
No, we are careful to abide by rules; yet the Rule Giver says rules kill. The rules (laws) weren't the problem. Inadequate flesh was. The rules of God---His commandments---which call us to go into all the world, to love one another, etc.---are hampered by our "laws" of why we can't, shouldn't, and couldn't when He has freed us to go for it!
In my very frail way I have made an effort to point out that the statement in that the law (in general) kills and the Spirit gives life is in fact a God-given statement. Liberty is a biblical calling; yet it makes us so uneasy we feel the need to explain exactly what we mean for we are barely free to accept it. Thus, the church is quite hampered (killed and dead) because we have focused on obeying rules of restriction instead of laws of liberty.