There are several passages of Scripture that give a succinct summary of the Christian life. Philippians 3:3 is one, 1 Thessalonians 1:9–10 is another, Galatians 2:20 too. I find myself thinking about a lot about another, 2 Corinthians 4:6–7.
In 2 Corinthians 3:7–18, Christ is contrasted to Moses. We remember the story of Moses speaking with God – when he came back down from the mountain, his face shone and he didn't realize it (Exodus 34:29–35). The children of Israel had Moses cover his face with a veil so that they could look at him. Now the glory of God is shining in the face of Jesus Christ. Unlike Moses, we are to look on Him without a veil. And when we do, His glory transforms us.
In 2 Corinthians 4:6–12, we have something Exodus didn't talk about: when we've been gazing at the glory of the Lord with unveiled faces, then God shines that same glory out of our hearts. It's the God who commanded light to shine out of darkness – the God who needs nothing to work with – who does this. He shines the "light of the knowledge of the glory of God in the face of Jesus Christ" out of our hearts.
Paul does that a lot, he talks about "the God who..." I've learned to pay attention to those small phrases, because they reveal an awful lot about the point the passage is making. Here it's the God who doesn't need any raw materials: He brought light out of darkness.
It's not mentioned in these verses, but we might pause a moment and consider that the first time God commanded light to shine out of darkness it didn't cost Him anything. He is God, He spoke and it was done. But in shining the light out of darkness in our hearts, the cost to Him was tremendous. It cost His Son.
2 Corinthians 4:7 goes on to say that God has deliberately put this treasure – the light of the knowledge of the glory of God in the face of Jesus Christ – in earthen vessels. He has chosen an entirely inappropriate vessel for His treasure. Why? Because He wants to be sure that we realize it's of God and not of us.
The passage doesn't actually mention the story of Gideon (Judges 7:16–21), but there are some striking parallels. First, we find that God carefully reduced the number of Gideon's men until they were down to 300 (Judges 7:1–7). We find, too, that God explicitly told Gideon why: He wanted there to be no question that it was He who brought victory, not strength of arms (Judges 7:2). Second, the weapons in the hands of Gideon's men were trumpets and torches (lights) hidden in earthen vessels (Judges 7:16).
We realize Gideon's plan was to reveal the torches not by lifting the vessels off the torches, but by breaking the earthen vessels. This is precisely what 2 Corinthians 4:10–12 goes on to talk about. As death works in us, the life of Jesus (notice here it's not "Christ Jesus" nor "Jesus Christ", but "Jesus") is revealed in our mortal bodies.
Susan has pointed out (quite correctly) that we don't cease to exist. Christianity is not Buddhism: we are not striving to become nothing. I'm afraid sometimes it sounds like that's what I'm saying – it's not. 2 Corinthians 4:16 makes it clear: there is an outward man that is broken down as death works in us, but there is an inward man that is renewed by this same process.
We saw this same contrast in Romans 7:22–23. There is an inward man delighting in the Law of God, but there is a law of sin in my members. What's the conclusion to the conflict in Romans 7? There the man cries out, "Who shall deliver me out of this body of death?" (Romans 7:24). Romans 8 picks up this theme in v. 10, where we find that the body is dead because of sin, but the spirit life because of righteousness. Romans 8 goes on to resolve this conflict in v. 23: we groan now, awaiting the redemption of the body. The Son of God is coming to change our bodies to be like His body (Philippians 3:21).
If I may pause here a minute: our hope as Christians is the resurrection of our mortal bodies to immortality. Someone once quoted 1 Thessalonians 4:17 to me about a man who is now asleep in Christ, "he is ever with the Lord." Of course that's entirely wrong – that phrase is clearly talking about those who shall have been raised into immortality. The dead in Christ haven't been made perfect without us: they await the resurrection just like we do. Our hope is, in a sense, physical: we await the resurrection of our mortal bodies. We might actually make it to that resurrection without dying, but all who are in Christ will be raised in what the Lord Jesus called the "resurrection of life" (John 5:29).
But our bodies haven't been raised to immortality yet. In a sense, that's really what the Christian life is – it's the life of Jesus manifested in mortal flesh (2 Corinthians 4:11). It's all about treasure in earthen vessels. It's about God's power seen in bodies that have yet to be redeemed.
It's true that the old man has died and the new man doesn't have to. At the same time, we recognize that death is the tool God has chosen to reveal Christ in us (2 Corinthians 4:10–12). We see the same truth in Colossians 3:1–5, because we have died with Christ, we are called to put to death our members on the earth. It's not that we are called to die, but we all carry about with us things that need to be put to death (Romans 8:12–14).
When the Son of God comes to change our mortal bodies, we won't have those things any more: there'll be no need to put to death the deeds of the body. But until then, death works in us.