Charles H. Spurgeon on Psalms 140:3

Man consists of soul and body; the body is but the shadow, or at best but the bearer of the soul: it's the soul that bears God's image; it's the soul especially for which Christ died. Now, by how much the soul is more precious than the body, by so much are the helps more excellent, and the enemies more dangerous than the body's. The body is fed with meat; but it is perishing meat (1Co 6:13); but the food of the soul is the heavenly manna (Joh 6:27). Answerably, the enemies are more hurtful, for that that hurts or kills the body toucheth not the soul; but what hurts or kills the soul kills the body with it, and destroys the whole man. The conclusion is, that therefore the bane or poison of the soul is much more hideous, horrible and hateful than that of the body; and of that poison speaks the present Scripture: adders' poison is under their lips.

A strange text, some may say, and 'tis true; but it is the fitter for these strange times, wherein the poison both of soul and body so far prevails. The words do describe in part the malignant and malicious nature of the unregenerate and sinful man; and to that purpose are they cited by the apostle to the Romans (Ro 3:13). The asp is but a little creature; but not a little poisonous. So little a creature hath been the bane and death of many a great person; let one suffice for all. That royal and renowned Cleopatra, queen of Egypt, chose rather to die by the biting of two asps than to be carried in triumph at Rome by Augustus. The manner of their poisoning is this, —he that is bitten by the asp falls forthwith into a gentle sweat and a sweet sleep, and his strength and vital spirits decay and weaken by little till he die; thus the present pain is little, but the stroke is deadly. And even such stings are the tongues, and such swords the words of wicked men. And no marvel; for what can come but poisonous words and actions from them whose very inward nature is all poison within!

The poison of the soul is only sin, and this is like to poison in many respects. Poison, wherever it enters, stays not there, but diffuses itself all over the body, and never ceaseth till it has infected all. Such is the nature of sin; enter where it will it creeps from one member of the body to another, and from the body to the soul, till it has infected tim whole man; and then from man to man, till a whole family; and stays not there, but runs like a wildfire, from family to family, till it has poisoned a whole town, and so a whole country, and a whole kingdom. Woeful experience proves this true, both for Popish opinions, idle fashions, vain customs, and ill —examples of all sorts, which once set on foot, spread themselves over the politic body of church and commonwealth, like a gangrene or a leprosy over the natural body, or like a poison through all the blood. Poison, having entered anywhere, as it seeks to creep presently over all, so desires it especially to seize upon the heart; such a malice and pride lies in the malignant nature of it, that it aspires to the heart; and such a craft and cunning lurks in it, that having once entered, it creeps closely and unfelt till it gets to the heart; but having possessed itself of that sovereign part of man, then like a tyrant it reigns and rages, and infecting first the vital blood and noble parts, it diffuses itself over all and every part. And such is the nature of sin, the spiritual poison of the soul; enter where it will, it is tim heart it aims at, and it will never stay till it comes there. The truth of this is so clear that proofs are needless; for who knows not that the senses are but the doors or windows, but the heart is the throne, and the soul itself the seat of sin: and hence it is that Solomon advises, —"My son, keep thy heart with all diligence": Pr 4:23. —William Crashaw, in "The Parable of Poyson," 1618.