This is the classic "tension" in New Testament Scripture-that Christ would come back soon:(Rev. 22:12)
And, behold, I come quickly; and my reward is with me, to give every
man according as his work shall be
The apostles seemed to be convinced that He would return during their lifetime, when they asked Him:(Acts 1:6)
When they therefore were come together, they asked of him, saying,
Lord, wilt thou at this time restore again the kingdom to Israel?
This was not an unfounded question: remember, the words of the angel Gabriel, when he announced to Mary:(Luke 1:32-33)
He shall be great, and shall be called the Son of the Highest: and
the Lord God shall give unto him the throne of his father David:
33 And he shall reign over the house of Jacob for ever; and of his
kingdom there shall be no end.
This was followed up by Zacharias's declaration:(Luke 1:74-75)
That he would grant unto us, that we being delivered out of the hand
of our enemies might serve him without fear, 75 In holiness and
righteousness before him, all the days of our life.
Jesus Himself didn't seem to dispel the notion; for instance:(Matt. 10:23)
But when they persecute you in this city, flee ye into another: for
verily I say unto you, Ye shall not have gone over the cities of
Israel, till the Son of man be come.
and in Matt. 16:28:
Verily I say unto you, There be some standing here, which shall not
taste of death, till they see the Son of man coming in his kingdom.
What is further bewildering for His disciples is when they asked,(Matt. 24:1)
And as he sat upon the mount of Olives, the disciples came unto him
privately, saying, Tell us, when shall these things be? and what shall
be the sign of thy coming, and of the end of the world?
He lists a series of events, culminating with(Matt. 24:30-31)
And then shall appear the sign of the Son of man in heaven: and then
shall all the tribes of the earth mourn, and they shall see the Son of
man coming in the clouds of heaven with power and great glory.
31 And he shall send his angels with a great sound of a trumpet, and
they shall gather together his elect from the four winds, from one end
of heaven to the other
Then He says an amazing thing:(vs 34)
Verily I say unto you, This generation shall not pass, till all these
things be fulfilled
The only word of caution we seem to have from Jesus is Acts 1:7,
And he said unto them, It is not for you to know the times or the
seasons, which the Father hath put in his own power.
Based on this, the church throughout the ages has had the mindset that "It could happen at any time". The Ante-Nicene fathers(Justin Martyr, Irenaeus, Hyppolytus had all expressed that the Lord could return during their day; Irenaeus and Hyppolytus even conjectured how Daniel's 70 week prophecy and the Antichrist would be fulfilled during their lifetimes.
So it is with this same vein that we are to understand 1 Cor. 7:29. Why? Because Our Lord said so! The "Normal Christian Life" is consumed by the mind set that (Matt. 25:13)
Watch therefore, for ye know neither the day nor the hour wherein the
Son of man cometh.
This doesn't mean that we aren't to marry, be given in marriage, raise our children, or plan for our retirement. What it does mean is that we live each day being ready to meet Him, and any time of the day-either 24 hour(3rd or 4th watch-Mark 13:35) or era. This is the lesson of the Wise and Foolish Virgins of Matt. 25:1-13.
The context of 1 Cor. 7:29 is in vs. 31,
"for the fashion of this world passeth away."
Clearly, this is what Jesus taught, and Paul re-enforces what the mindset of a believer should be.