Oh, someone will say, you mean that if I become a Christian, or I walk as a Christian, I will live a very long life? Well you are forgetting, this is parables, comparisons, similes. No, the promise is not necessarily that you will literally have a long life.
Does a short life imply the Lord’s discipline? The operative word here is 'added'. And it suggests that our course can make a difference to the time span which the Lord has allotted to us. 1 Corinthians 11 hints of this where it is suggested that some people, some Christians, because of their inconsistent walk and their lack of self-examination have become such an embarrassment to the Lord that they have even been taken from this life, they sleep. Now immediately one ought to say that no-one should ever conclude that Christians who are taken home early or earlier than you would expect are therefore being taken because of an inconsistent life. We can soon dispel notions like that from our minds. We have only to look at the Murray M'Cheynes of this world to realise that the Lord sometimes operates a totally different principle and calls to himself some of the very choicest early. Obviously it follows therefore that a consistent walk will at least guarantee that we will never have to be removed because of embarrassment that we may cause. But then, people suggest other things. Obviously a consistent Christian ought to be somewhat less likely to suffer from the kind of heart condition which is brought about by the festering anxiety of a worldling, full of frustration and ill temper and who knows what else. But certainly one has to qualify this. I am sure there are a lot of heart conditions which are not produced by sin and by worldliness but it equally follows that a Christian who is living consistently should be delivered from those conditions that are produced by evil living or godless living. Then again, we qualify all these things because the word of God teaches that some people are called to bear suffering to prove the very grace of God, not because they have not been living as they should. So these things are full of mystery and yet the promise is true that God will add usefulness and even length of days to those who live consistently.