Let's look at how it seems he assumed Christ's return to be in his lifetime.
1Th 4:15 For this we declare to you by a word from the Lord, that we who are alive, who are left until the coming of the Lord, will not precede those who have fallen asleep.
1Th 4:16 For the Lord himself will descend from heaven with a cry of command, with the voice of an archangel, and with the sound of the trumpet of God. And the dead in Christ will rise first.
1Th 4:17 Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air, and so we will always be with the Lord.
1Th 4:18 Therefore encourage one another with these words.
Notice how he says "we who are alive, who are left" as though he is assuming he will be counted among those still alive when this happens. Think I am making too much of this verse? Let's keep looking.
1Co 7:26 I think that in view of the present distress it is good for a person to remain as he is.
1Co 7:29 This is what I mean, brothers: the appointed time has grown very short. From now on, let those who have wives live as though they had none,
1Co 7:30 and those who mourn as though they were not mourning, and those who rejoice as though they were not rejoicing, and those who buy as though they had no goods,
1Co 7:31 and those who deal with the world as though they had no dealings with it. For the present form of this world is passing away.
Apparently he felt "the appointed time" had grown so very short that he advised people not to marry, as though "the present form of the world" would pass away within a lifetime. In other words, "You won't be around much longer, so don't be concerned with things like marriage."
Now if you aren't tracking with me yet, thinking to yourself that these passages teach us the doctrine of Christ's imminent return, that we and Christians for the past 2000 years ought to have lived expecting Christ to return "any moment now", then answer me this: Are you living as Paul teaches in 1 Cor. 7:29-31? I mean, really consider yourself; do you really buy as though you have no goods? Do you really live with your wife as though you had none?
Even if you don't buy anything I say, if you believe Paul is here teaching Christ's imminent return today, I encourage you to live as Paul advises. (And if you've read here before at all, I would suggest that this lifestyle looks a lot like simple living.)
For those of you that, like me, wonder if Paul was wrong in making an assumption, or if you would just humor me, what then are the implications? The danger, I feel, is to start doubting Paul's teachings. I mean, while it may be entirely true that a married man's interests are divided (1 Cor. 7:33-34), it is tempting to dismiss the advice in this passage because Paul based it upon a wrong assumption. This is why verse 25 is so important, "I have no command from the Lord, but I give my judgment as one who by the Lord's mercy is trustworthy." So Paul's advice here is just that--his own advice. And if I think he is wrong, then I am not calling into question the inerrancy of Scripture, but the inerrancy of Paul. So the implication of Paul being wrong is that we can doubt and disagree with the things he says are from him and not the Lord without casting doubt upon Scripture.
What other implications are there? Would Paul have lived differently? Have you ever wondered?