"Therefore we do not lose heart. Though outwardly we are wasting away, yet inwardly we are being renewed day by day. For our light and momentary troubles are achieving for us an eternal glory that far outweighs them all. So we fix our eyes not on what is seen, but on what is unseen, since what is seen is temporary, but what is unseen is eternal."
In his second letter to the Corinthians, Paul expresses his relief and joy that the Corinthians had received his “severe” letter (now lost) in a positive manner. That letter addressed issues that were tearing the church apart, primarily the arrival of self-styled false apostles who were assaulting Paul’s character, sowing discord among the believers, and teaching false doctrine. They appear to have questioned his veracity, his speaking ability, and his unwillingness to accept support from the church at Corinth. There were also some people who had not repented of their licentious behavior.
It was an opinion among the Jews that even spirits stood in need of continual renovation. They say that "God renews the angels daily, by putting them into the fiery river from which they proceeded, and then gives them the same name they had before." And they add, that in like manner he renews the hearts of the Israelites every year, when they turn to him by repentance. It is a good antidote against the fear of death to find, as the body grows old and decays, the soul grows young and is invigorated.
Matthew 22:36-40 “Teacher, which is the greatest commandment in the Law?” Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ This is the first and greatest commandment. And the second is like it: ‘Love your neighbor as yourself.’ All the Law and the Prophets hang on these two commandments.”
2 Corinthians 4:16-18 relates to Matthew 22:36-40 based on its ability to use love and faith in the same context. When Corinthians speaks to us to eternally remain strong when we are deteriorating on the outside correlates to how we must always "love The Lord" even through the times when our faith or ability to love is hindered. We are often surrounded by situations and scenarios that keep us from seeking God or having faith in his works, and limits us from actually seeing the truth in His word.
Through Matthew we are told how we must spend our lives loving The Lord with not only our words, but our minds. Also, loving our neighbor even in the most difficult situations. Corinthians expands on this by saying that we must keep humble, strong in faith, and remain in Him when the faith we know is "unseen" or the momentary troubles are restricting us from knowing what we should do to love our neighbors and God. In both verses, we can only achieve eternal life through the word of a God considering loving Him and always trusting in the faith we have.
This verse fits into the chapter because in the verse Paul talks about staying faithful to God and understanding that God's eternal glory outweighs all our troubles and suffering, the main theme of the chapter is troubles and faithfulness. What is happening in this chapter is Paul is writing about what we will go through in our lives, there will be things that will try to knock us off of our course and make us turn away from God. Paul reassures us that we renew each day by God's glory. His glory outweighs all of those troubles. He also tells us not to focus on what is seen but what is unseen as well, because God works in our lives and it's not always seen while doing so. Paul writes this to inform the people of Corinth of this good news.
When I have seen by Time's fell hand defac'd The rich-proud cost of outworn buried age; When sometime lofty towers I see down-razed And brass eternal, slave to mortal rage; When I have seen the hungry ocean gain Advantage on the kingdom of the shore, And the firm soil win of the wat'ry main, Increasing store with loss, and loss with store; When I have seen such interchange of state,
Or state itself confounded to decay; Ruin hath taught me thus to ruminate -- That Time will come and take my love away. This thought is as a death, which cannot choose But weep to have that which it fears to lose.
Now that I have seen time’s terrible hand deface the costly and splendid monuments of buried men from ages past, and once-lofty towers torn down; now that I have seen even hard brass subject to perpetual destruction by human beings; now that I have seen the hungry ocean swallow up the land and firm land seize territory from the ocean, so that each one’s loss is the other’s gain; now that I have seen that all things constantly change into something else or fall into decay—all this destruction has taught me to think: The time will come in which time will take my love from me. This thought feels like death, and makes me weep over what I have that I’m afraid of losing.