There are three reasons why Christ has received this name, "The Sun of Righteousness."
I. He gives Light. He lived in the world and was the Sun of the world. The great and wise of every land poets, philosophers, inventors, teachers may be compared to the stars, but He alone is the Sun. Why? Because He brought knowledge to us infinitely greater and more precious than all other knowledge besides. He made God manifest to men. He revealed the means of saving the soul from death. He discovered to men a new world the better land above the stars.
II. He gives Life. He is the life of the soul. Sin is its death. Sin is like the frost which withers and desolates the land. But Christ is like the summer sun that drives the cold away, and restores life and beauty to the dead earth. He releases our souls from the mortal hold of sin, and bids them live.
III. He gives Joy. Darkness has always been the symbol of sorrow, and sunshine of joy. Christ is the Sun of the soul, because He fills it with joy. There is no joy in the world like that which Christ gives. How does Christ give joy? It is by giving love and by giving salvation. Not only does He shine on those who love Him in this world, but He stands in the zenith of the better world and floods it for evermore with joy.
J. Stalker, The New Song,p. 105.
References: Malachi 4:2. Spurgeon, Sermons,vol. xvii., No. 1020; vol. xxv., No. 1463; Ibid., My Sermon Notes: Ecclesiastes to Malachi,p. 390; Clergyman's Magazine,vol. xi., p. 217; Homiletic Quarterly,vol. i., p. 88; Preacher's Monthly,vol. v., p. 360.