God’s justice and His power are inseparable; ’tis in vain we invoke His power in an unjust cause. We are to have our souls pure and clean, at that moment at least wherein we pray to Him, and purified from all vicious passions; otherwise we ourselves present Him the rods wherewith to chastise us; instead of repairing anything we have done amiss, we double the wickedness and the offence when we offer to Him, to whom we are to sue for pardon, an affection full of irreverence and hatred. Which makes me not very apt to applaud those whom I observe to be so frequent on their knees, if the actions nearest to the prayer do not give me some evidence of amendment and reformation.

[Sa justice & sa puissance sont inseparables: Pour neant implorons nous sa force en une mauvaise cause: Il faut avoir l’ame nette, au moins en ce moment, auquel nous le prions, & deschargée de passions vitieuses: autrement nous luy presentons nous mesmes les verges, dequoy nous chastier. Au lieu de rabiller nostre faute, nous la redoublons ; presentans à celuy, à qui nous avons à demander pardon, une affection pleine d’irreverence & de haine. Voyla pourquoy je ne louë pas volontiers ceux, que je voy prier Dieu plus souvent & plus ordinairement, si les actions voisines de la priere, ne me tesmoignent quelque amendement & reformation.]

Michel de Montaigne (1533–1592) French essayist

Essays, Book 1, ch. 56 (1.56), “Of Prayers [Des prieres]” (1572–1580) [tr. Cotton/Hazlitt (1877)]
    (Source)

This passage was in the 1st (1580) edition.

(Source (French)). Other translations:

His justice and power are inseparable. It is but in vaine to emplore his power in a bad cause. Man must have an unpolluted soule when he praieth (at least in that moment he addresseth himselfe to pray) and absolutely free from all vicious passions; otherwise we our selves present him the roddes to scourge us withall. In liew of redressing our fault, we redouble the same, by presenting him with an affection fraught with irreverence, sinne, and hatred, to whom onely we should sue for grace and forgivenesse. Loe-heere, why I doe not willingly commend those Pharisaicall humours, whom I so often behold, and more then ordinary, to pray unto God, except their actions imediately preceding or succeeding their prayers witnesse some shew of reformation or hope of amendment.
[tr. Florio (1603)]

God's justice, and his power, are inseparable; and therefore in vain we invoke his power in an unjust cause: we are to have our souls pure, at that moment at least, wherein we pray to him, and free from all vicious passions, otherwise we ourselves present him the rods wherewith to chastise us. Instead of repairing any thing we have done amiss, we double the wickedness and the offence, whilst we offer to him, to whom we are to sue for pardon, an address full of irreverence and hatred. Which makes me not very apt to applaud those whom I observe to be so frequent on their knees, if the actions before and after the prayer, do not give me some evidence of reformation and amendment.
[tr. Cotton (1686)]

His justice and his power are inseparable. To no purpose do we implore his might in a bad cause. Our souls must be clean -- at all events at the moment when we pray to him -- and free of evil passions; otherwise we ourselves present to him the rods wherewith to chastise us. Instead of redressing our offence, we redouble it, offering to him whose forgiveness we have to ask, a heart full of irreverence and hatred. This is why I do not readily praise those whom I see praying to God most frequently and most commonly, if their actions following the prayer do not witnes to me some amendment and reformation.
[tr. Ives (1925)]

His justice and his power are inseparable. For naught do we implore his power in an evil cause. We must have our soul clean, at least in that moment in which we pray to him, and rid of vicious passions; otherwise we ourselves present to him the rods with which to chastise us. Instead of redressing our fault, we redouble it when we offer feelings full of irreverence and hatred to him whom we are to ask for pardon. That is why I am not inclined to praise those whom I see praying to God most often and habitually, if the actions surrounding the prayer do not show me evidence of some amendment and reform.
[tr. Frame (1943)]

God’s power and his justice are inseparable. If we implore him to use his power in a wicked cause it is of no avail. Our soul must be pure, at least for that instant when we make our prayer, free from the weight of vicious passions; otherwise we offer him rods for our own chastisement. Instead of amending our faults we redouble them by offering God (from whom we ought to be begging forgiveness) emotions full of irreverence and hatred. That is why I do not approve of those whom I see praying to God frequently and regularly if deeds consonant with their prayers do not bear me witness of some reformation and amendment.
[tr. Screech (1987)]