In 1 Corinthians 2-3, Paul warns the Corinthian church about confusing the wisdom of man with the wisdom of God. Human wisdom places the wrong value on things, so that we end up building our lives out of things that are worth no more than "wood, hay or straw" in the eternal reality (3:12). Such things, while perhaps satisfying in this life, will not survive the fires of God's testing. If our lives consist of nothing more than that which the world prizes, we will arive in heaven as refugees from the collapse of the world, with nothing to show but our skin for all that we have done on earth (3:13-15).
If our own self interest is not reason enough for us to be careful to invest ourselves in the things that God prizes, Paul in verse 16 flips things over to show us God's perspective on our labors. By sending His Holy Spirit into believers, God makes us into His temples, places where His very nature dwells. This is no small thing, indeed we are as significant as anything anywhere. As God's temple each one of us is sacred and set apart for Him. We are not free to fill our lives with trashy thoughts, junky behaviors, or warped affections any more than we would be free to fill the White House with compost or trash. Your body is not your home but God's, so if you mistreat it you insult the God who dwells there and can expect that he will respond accordingly.
The problem is that because human wisdom prizes what God considers to be trash, by relying on it we deceive ourselves into becoming fools in God's sight (v19). We are far better off being thought wise by God even if it means the world thinks us simpletons. Indeed, the world prizes craftiness, for all human wisdom is based on self-interest even though all efforts to exalt ourselves are ultimately futile. While the Corinthians were concerned to make themselves look good to others by affiliating with the "right" preacher, God evaluated them according to what they did with the preacher's teachings. The Corinthians worried about who their mailman was, while God wanted to know what they did with the million dollar check that he delivered.
This is not, as Paul points out, to say that the preachers are of no significance (4:1), but rather to see them in the light that God sees them, as messengers entrusted with God's message to them. As such, their business is not to impress the Corinthians, or even to satisfy themselves, but to satisfy God that they have properly handled the message He gave them to deliver. External evidence won't by itself tell us what God thinks, anymore than the color of the mailman's uniform will tell us about the quality of the job he's doing. It is when God Himself reveals His assessment of his messengers' labors that we'll be able to accurately judge how well they have done.
This does not mean that we will never be able to judge if a believer is doing well or not, for there are some truths that God has already revealed by which we can judge each others' behavior and teachings. However there are many more criteria by which people are prone to judge each other, and it is in these areas that the wise man or woman will wait until the appointed time, to discover what God has to say about each of us. As Jesus has told us in the parable of the talents, we are all given different gifts, and will be judged not on how our fruit compares with the fruit of those around us, but on how fruitfully we have used the gifts we were given. The widow's mite was the greater gift than the wealth of the Pharisees because Jesus is more interested in the quality of the heart that gives than the amount given. So we need to beware of judging those who labor on God's behalf; given the resources and calling that God has given them, they could be doing far better or worse in God's sight that it appears to us and we'd never know it until the end.