What does 2 Corinthians 4:18 mean?

Paul is describing the reason for his refusal to lose heart in the face of suffering for his faith in Christ. He wrote in the previous verse that his current suffering, though nearly unbearable at times (2 Corinthians 1:8), can't even be compared to the far weightier glory of eternity.

Now he adds that this perspective requires a focus on what cannot be seen in this life, meaning the spiritual world. The things that are visible to humans in this life are here for just a moment and then gone. Paul describes them as transient. This is true in two senses. Obviously, anything that exists in this world lasts a very short time in comparison with those things that last forever. Secondly, though, human lives are far shorter, yet, than human history. What is visible to us will come and go very quickly.

The invisible God, though, is "eternal," meaning "outside of time." Whatever exists with Him in the spiritual world will never end. Paul is able to keep his focus on the glories of eternity by keeping his inner focus on what is eternal. This allows him to endure suffering in this life that is "light" and "short" in comparison to the glories and pleasures of eternity to come (Hebrews 11:14–16).

Paul commanded believers to exercise the same focus in Colossians 3:1–4, "Since, then, you have been raised with Christ, set your hearts on things above, where Christ is, seated at the right hand of God. Set your minds on things above, not on earthly things. For you died, and your life is now hidden with Christ in God. When Christ, who is your life, appears, then you also will appear with him in glory. (NIV)"