In fact, even were it not inhuman to trouble another man’s conscience, even were there none of the bad effects that spring up by the thousands, one would have to be deranged to engage in such a purpose. He who tries to make me change my religion only does so, presumably, because he would never change his own, even were someone to try to force him to do so; so how can he find it strange that I should refuse to do something that he himself would not do, even perhaps were he offered the world as his empire?

[Car, enfin, quand il n’y auroit pas de l’inhumanité à affliger la conscience des autres, quand il n’en résulteroit aucun des mauvais effets qui en germent à milliers, il faudroit être fou pour s’en aviser. Celui qui veut me faire changer de religion ne le fait sans doute que parce qu’il ne changeroit pas la sienne, quand on voudroit l’y forcer : il trouve donc étrange que je ne fasse pas une chose qu’il ne feroit pas lui-même, peut-être pour l’empire du monde.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Persian Letters [Lettres Persanes], Letter 86, Usbek to Mirza (1721) [tr. Mauldon (2008), # 83]
    (Source)

(Source (French)). Alternate translations:

For, in short, though there was nothing of inhumanity in forcing the consciences of others; though it occasioned none of those ill effects which spring up from it by thousands; a man must be a fool to offer at it. He that would have me change my religion, does it, no doubt, because he would not change his own if he were to be forced to it: so that he wonders I will not do a thing, which perhaps he would not do himself for the empire of the universe.
[tr. Ozell (1760 ed.)]

For indeed, if there was nothing of inhumanity in forcing the conscience of another, though there did not arise from it any of those bad effects which spring from it by thousands, it would be folly to advise it. He who would have me change my religion, no doubt, desires me to do so, because he would not change his own if he were forced to do it: he yet thinks it strange, that I will not do a thing which he himself would not do, perhaps, for the empire of the world.
[tr. Floyd (1762), # 85]

In conclusion, even if there were no inhumanity in distressing the consciences of others, even if there did not result from such a course any of the evil effects which do spring from it in thousands, it would still be foolish to advise it. He who would have me change my religion is led to that, without doubt, because he would not change his own although force were employed; and yet he finds it strange that I will not do a thing which he himself would not do, perhaps for the empire of the world.
[tr. Davidson (1891)]

For, in fine, even if the attempt to trouble the conscience of our neighbor was not in itself inhuman, if the manifold evil effects which spring from it had no existence, the mere contemplation of such a course would be an evidence of mental unsoundness.
The man who would have me change my religion does so doubtless because he would never change his own, no matter what force was brought to bear upon him: yet he thinks it strange that I should refuse to do a thing he would not himself do for the empire of the whole world!
[tr. Betts (1897)]

Finally, even if it were not inhumane to afflict another’s conscience, and even if there did not result from such an act those bad effects which spring up by the thousands, it would still be foolish to advise it. Whoever would have me change my religion doubtlessly acts as he does because he would not change his, however he was forced; yet he finds it strange that I will not do something which he would not do himself, perhaps for the entire world.
[tr. Healy (1964), # 85]

Finally, even if it were inhumane to make such an assault on the consciences of others, with its thousands of self-replicating evil consequences, one would have to be mad to advise it. The man who wants me to change my religion does so only because he would never change his, no matter how much he was forced. And thus he thinks it strange that I will not do a thing he would not do for all the kingdoms of the world.
[tr. MacKenzie (2014), # 85]