The Duty of Gladness.
I. It is of the very nature of a duty that it is in our power to perform it; and so with this one, the very fact of its being laid upon us proves that we may, if we will, obey it. And therefore this at once disposes of those who would be inclined to say that gladness does not depend on ourselves, that it is the privilege of the few only to be gay, and of those few only under peculiar circumstances; and that it is as vain to tell people to be merry and joyful as to tell them to be tall or short, or strong or handsome. There is always a disposition to make every thing in our Christian life dependent on circumstances, and to make excuses for this or that sin or shortcoming, by blaming circumstances and not ourselves. Once begin with the perilous doctrine that men are what they are made, and that we cannot help our lapses because of the taint and defects in our nature, and we open the door to excuses for every kind of enormity.
II. Just as we get nearer to our true selves, the fresher and purer, and wiser and truer our souls become, the more food shall we find for joy; and because, as the pure soul finds life glad, and so gladness reacts upon the soul and tends to make it pure, so this is the reason why the Apostle tells us to rejoice; for joy tends to cleanse the heart and banish thought of sin and misery, and wars against the useless recollection of sorrows that are gone, and of errors that cannot now be retrieved, and of troubles that may be temptations to murmur, but which by all the murmurs in the world can never be as though they were not. Sin slays gladness, and sin alone; and this is the awful part of the curse on sin, that it robs us of our inheritance of delight, and is a bar to our hearty joy. But to those who are trying to realise that they are Christ's redeemed ones, and who live in the habitual remembrance that God is their Father, joy need not be and ought not to be hard.
A. Jessopp, Norwich School Sermons,p. 226.
Reference: 1 Thessalonians 5:16. Spurgeon, Sermons,vol. xxxii., No. 1900.