It was the saying of a precious saint, that he was more afraid of his duties than of his sins, for the one made him often proud, and the other made him always humble. It was good counsel Luther gave, Cavendum est non solum a peccatis, sed a bonis operilus, we must take heed not only of our sins, but of our good works. Duties can never have too much diligence used about them, nor too little confidence placed in them. They are good helps, but bad saviours. It is necessary we do them, but it is dangerous to rely upon them. If the devil cannot dissuade us from performing religious duties, then his next work will be to persuade us all he can to rely upon them, to make saviours of them, because this will as much gratify Satan, and as certainly ruin our souls, as if we had wholly neglected them. O man! thine own righteousness rested in, will as certainly and eternally undo thee as the greatest and foulest enormities. This soul-sickness is that spiritual idolatry that will undo thee, for thou makest thyself a saviour, and thy duties a saviour, and sayest of thy duties as they did of the golden calf, ‘These are the gods that brought thee out of the land of Egypt.’ Open wickedness, open idolatry, slays her thousands, but secret idolatry, a secret resting upon duties, slays her ten thousands. Multitudes bleed inward by this disease, and die for ever. Open profaneness is the broad dirty way that leads to hell, but religious duties rested in, is as sure a way, though a cleaner way to hell. Profane persons and formal professors shall meet at last in one hell...  Oh that these three last things might work you to be more in love with free grace than ever, and to be more in love with the righteousness of Christ than ever, and to be more in love with the covenant of grace than ever.1

-Thomas Brooks, A Cabinet of Choice Jewels, or, A Box of Precious Ointment, pp. 495-496 & 497, published in 1669.

Rodin's The Gates of Hell
  1. A modern reading of the above quote: “It was the saying of a precious saint‚ that he was more afraid of his duties than of his sins; for his duties often made him proud‚ but his sins always made him humble. It was good counsel Luther gave, We must take heed not only of our sins‚ but of our good works. Duties can never have too much diligence used about them‚ nor too little confidence placed in them. They are good helps‚ but bad saviors. It is necessary we do them‚ but it is dangerous to rely upon them. If the devil cannot dissuade us from performing pious duties‚ then his next work will be to persuade us to rely upon them, to make saviors of them; because this will as certainly ruin our souls, as if we had wholly neglected them. Resting in your own righteousness, will as certainly and eternally undo you‚ as the greatest and foulest atrocities! Open wickedness slays her thousands‚ but a secret resting upon duties, slays her ten thousands! Open profaneness is the broad dirty way which leads to hell; but trusting in pious duties is as sure a way, though a cleaner way to hell. Ungodly people and formal professors shall meet at last in the same hell... Now, let all these things work you to renounce your own righteousness‚ and to take sanctuary alone in the pure, perfect, and most glorious righteousness of Jesus Christ, and in the free grace of God.” ↩︎