This light momentary affliction is preparing for us an eternal weight of glory beyond all comparison, as we look not to the things that are seen but to the things that are unseen. For the things that are seen are transient, but the things that are unseen are eternal.
2 Corinthians 4:17-18 ESV
Later in the same letter he tells us what some of these light afflictions were.
Are they servants of Christ? I am a better one—I am talking like a madman—with far greater labors, far more imprisonments, with countless beatings, and often near death. Five times I received at the hands of the Jews the forty lashes less one. Three times I was beaten with rods. Once I was stoned. Three times I was shipwrecked; a night and a day I was adrift at sea; on frequent journeys, in danger from rivers, danger from robbers, danger from my own people, danger from Gentiles, danger in the city, danger in the wilderness, danger at sea, danger from false brothers; in toil and hardship, through many a sleepless night, in hunger and thirst, often without food, in cold and exposure. And, apart from other things, there is the daily pressure on me of my anxiety for all the churches.
2 Corinthians 11:23-28 ESV
This letter was written before the shipwreck described in Acts 28, so Paul was shipwrecked at least four times during his life. It is possible that after the events recorded in Acts he experienced even more afflictions like the ones he describes here. Whatever happened to him, it didn’t change his attitude toward his suffering. Here is what he said in his final letter, written while he was in prison waiting for his execution.
For I am already being poured out as a drink offering, and the time of my departure has come. I have fought the good fight, I have finished the race, I have kept the faith. Henceforth there is laid up for me the crown of righteousness, which the Lord, the righteous judge, will award to me on that day, and not only to me but also to all who have loved his appearing.
2 Timothy 4:6-8 ESV
He had not abandoned his belief that the glory which lay in his future made the sufferings of his present life worthwhile.
If the sufferings of a Christian prepare him for future glory, what effect do the pleasures of an unsaved person have? Suppose someone rejects God and lives solely for his own pleasure. He might live a life of happiness and success in everything he does, but when he comes to the end of his life he might say something like this: “This light momentary pleasure is preparing for me an eternal weight of judgment beyond all comparison.” The life we live now is only the starting point of an eternal existence. It is the most important part of our existence because the decisions we make now will have eternal consequences.
In the previous paragraph I spoke of eternal existence, not eternal life. Here is how Jesus defined eternal life.
And this is eternal life, that they know you, the only true God, and Jesus Christ whom you have sent.
John 17:3 ESV
Those who have not turned from their sins and put their faith in Christ are already spiritually dead and if they continue in unbelief they will remain dead for all eternity.