(1.) The way of righteousness is light (v. 18): The path of the just, which they have chosen, and in which they walk, is as light; the light shines on their ways (Job 22 28) and makes them both safe and pleasant. Christ is their way and he is the light. They are guided by the word of God and that is a light to their feet; they themselves are light in the Lord and they walk in the light as he is in the light. [1.] It is a shining light. Their way shines to themselves in the joy and comfort of it; it shines before others in the lustre and honour of it; it shines before men, who see their good works, Matt 5 16. They go on in their way with a holy security and serenity of mind, as those that walk in the light. It is as the morning-light, which shines out of obscurity (Isa 58 8, 10) and puts an end to the works of darkness. [2.] It is a growing light; it shines more and more, not like the light of a meteor, which soon disappears, or that of a candle, which burns dim and burns down, but like that of the rising sun, which goes forward shining, mounts upward shining. Grace, the guide of this way, is growing; he that has clean hands shall be stronger and stronger. That joy which is the pleasure of this way, that honour which is the brightness of it, and all that happiness which is indeed its light, shall be still increasing. [3.] It will arrive, in the end, at the perfect day. The light of the dayspring will at length be noon-day light, and it is this that the enlightened soul is pressing towards. The saints will not be perfect till they come to heaven, but there they shall themselves shine as the sun when he goes forth in his strength, Matt 13 43. Their graces and joys shall be all consummate. Therefore it is our wisdom to keep close to the path of the just.

Matthew Henry Commentary on the Whole Bible (Concise) (Matthew Henry)

Verses 14-27 The way of evil men may seem pleasant, and the nearest way to compass some end; but it is an evil way, and will end ill; if thou love thy God and thy soul, avoid it. It is not said, Keep at a due distance, but at a great distance; never think you can get far enough from it. The way of the righteous is light; Christ is their Way, and he is the Light. The saints will not be perfect till they reach heaven, but there they shall shine as the sun in his strength. The way of sin is as darkness. The way of the wicked is dark, therefore dangerous; they fall into sin, but know not how to avoid it. They fall into trouble, but never seek to know wherefore God contends with them, nor what will be in the end of it. This is the way we are bid to shun. Attentive hearing the word of God, is a good sign of a work of grace begun in the heart, and a good means of carrying it on. There is in the word of God a proper remedy for all diseases of the soul. Keep thy heart with all diligence. We must set a strict guard upon our souls; keep our hearts from doing hurt, and getting hurt. A good reason is given; because out of it are the issues of life. Above all, we should seek from the Lord Jesus that living water, the sanctifying Spirit, issuing forth unto everlasting life. Thus we shall be enabled to put away a froward mouth and perverse lips; our eyes will be turned from beholding vanity, looking straight forward, and walking by the rule of God's word, treading in the steps of our Lord and Master. Lord, forgive the past, and enable us to follow thee more closely for the time to come.