Thomas Shepard preaches about the insufficiency of relying on our own good deeds for salvation, emphasizing that even our best actions are tainted with sin and cannot save us. He warns that no amount of good works can make up for past sins or secure our salvation, as one sin can lead to our downfall. Shepard highlights the importance of not just performing duties, but humbly offering them to Jesus Christ, acknowledging that any goodness in our actions comes from Him alone.

Therefore behold the insufficiency of all duties to save us; which will appear in these three things which I speak, that you may learn hereafter never to rest in duties:— First. Consider, your best duties are tainted, poisoned, and mingled with some sin, and therefore are most odious in the eyes of a holy God, (nakedly and barely considered in themselves,) for, if the best actions of God's people be filthy, as they come from them, then, to be sure, all wicked men's actions are much more filthy and polluted with sin; but the first is true—"All our righteousnesses are as filthy rags;" for as the fountain is so is the stream; but the fountain of all good actions (that is, the heart) is mingled partly with sin, partly with grace; therefore every action participates of some sin, which sins are daggers at God's heart, even when a man is praying and begging for his life; therefore there is no hope to be saved by duties. Secondly. Suppose you could perform them without sin; yet you could not continue in doing so. (Is. 40:6,) "All flesh and the glory thereof is but grass." So your best actions would soon wither if they were not perfect; and if you cannot persevere in performing all duties perfectly, you are forever undone, though you should do so for a time, live like an angel, shine like a sun, and, at your last gasp, have but an idle thought, commit the least sin, that one rock will sink you down even in the harbour, though never so richly laden. One sin, like a penknife at the heart, will stab you; one sin, like a little burning twig in the thatch, will burn you; one act of treason will hang you, though you has lived never so devoutly before, (Ezek. 18:24;) for it is a crooked life when all the parts of the line of your life be not straight before Almighty God. Thirdly. Suppose you should persevere; yet it is clear you have sinned grievously already; and do you think your obedience for the time to come can satisfy the Lord for all those previous obligations, for all those sins past? Can a man that pays his rent honestly every year satisfy hereby for the old rent not paid in twenty years? All your obedience is a new debt, which can not satisfy for debts past. Indeed, men may forgive wrong and debts, because they be but finite; but the least sin is an infinite evil, and therefore God must be satisfied for it. Men may remit debts, and yet remain men; but the Lord having said, "The soul that sins shall die," and his truth being himself, he can not remain God, if he forgive it without satisfaction. Therefore duties are but rotten crutches for a soul to rest upon. But to what end should we use any duties? Can not a man be saved by his good prayers, nor sorrows, nor repentings? Why should we pray any more then? Let us cast off all duties, if all are to no purpose to save us; it is as good to play for nothing as to work for nothing. Though your good duties can not save you, yet your bad works will damn you. You are, therefore, not to cast off the duties, but the resting in these duties. You are not to cast them away, but to cast them down at the feet of Jesus Christ, as they did their crowns, (Rev. 4:10,11,) saying, If there be any good or graces in these duties, it is yours, Lord; for it is the prince's favor that exalts a man, not his own gifts: they came from his good pleasure.