In 1762 a young man committed suicide in his home in Toulouse. This would be a tragedy for any family, but the tragedy of the Calas family would be even bigger. Since the whole family was at home at the time when the son commited suicide, and, more importantly, since they were Huguenots, the Catholic mob accused them of killing their son. Jean Calas, the father was convicted and was tortured to death on the wheel. The sons were forced into a monastery (prison), the daughters were forced into convents (prison), while the mother lost all her positions and was to end her life in poverty.
While the French Catholics were approving of this trial or (at best) looking away, one man was disgusted by it all. This man was Voltaire, and he decided to write a treatise to promote religious tolerance. During his stay in England (1726-1728), Voltaire became acquainted with philosopher John Locke. In his Letter on Toleration (1689) Locke argued that no church could have a legitimate claim to power. Civilians agreed to a social contract whereby the sovereign would gain legislative and executive powers, so civilians only had to comit themselves to the state - churches had no right to claim power over civilians, since this power would be unfounded.
In the Treatise on Toleration, Voltaire decides to pick another route to toleration. He writes as if he were a Catholic (90% of France were Catholics, so this is the audience he aims at) and from this point he argues for toleration for all the religious minorities (especially the Huguenots). He wrote this in a time of heavy persecution by the French churches and a mass exodus of Huguenots to Holland and England.
The text itself is not really one whole; each chapter of the 25 is short and deals with a different subjects. There are some main themes in this treatise, but Voltaire was no philosopher or scientist and this shows in his text. It would have been better if he picked a structure and made some choices in what to include and what to exclude.
In broad outlines: he uses the Calas execution as a motivation to write this treatise. Next, he traces the attitude that lead to this particular execution - but more importantly: religious fanaticism in general - to the Reformation. After this looks at the state of religious tolerance in ancient Greece, the Roman Empire and early Christendom. He concludes that these times saw religious tolerance and that the only intolerance came from christians (nowadays, we would call the martyrologies of early Christendom fake news).
After this brief historical overview, Voltaire states that religious intolerance leads to the cessation of Reason and to abuse. He then asks the reader this important question: what does Scripture say about tolerance? He shows that, according to books of the Old Testament, the Jews knew long periods of tolerance and according to the New Testament, the message of Jesus is tolerance - and no compulsion. I think this last part is important, since fanatical christians who see Christ as saviour, would do well to ponder this fact. He underlines this important point with the plea for tolerance of some famous writers (churchfathers, philosophers, etc.).
At this point the book becomes a collection of short statements. He tries to show with a dialogue how unreasonable intolerance is and uses a letter, supposedly written by a Jesuit to encourage attacks on non-Jesuits, to show the danger in practice. Besides this, Voltaire reasons from a practical standpoint: the exodus of Huguenots makes Holland and England militarily and economically stronger, so intolerance leads to a weaker nation.
The most important message is contained in chapter 18, which basically summarizes the whole treatise: is intolerance justified? Only to destroy intolerance, according to Voltaire. This reminded me of the famous quotation of Karl Raimund Popper, who claimed that we should be "intolerant of intolerance". The hidden message of this chapter is, of course, that the state should actively exterminate religious intolerance and fanaticism (this clearly is Locke's influence on Voltaire).
Voltaire endeavors to establish universal tolerance. Posing as a Catholic, he hoped to convince the Catholic French majority that this was the best option. I'm not in a position to judge if Voltaire succeeded in this, but my guess is that the horrors of the Terror during the French Revolution were (partly) a reaction to centuries of Catholic persecution, censure and oppression. Intolerant people will reap what they sow.
I'm also not in the position to make claims about the importance and influence of this treatise; as a book I found it unstructured and at times too random (as if it lacks a main objective), but as a plea for (religious) tolerance, the contents are not less important. It looks like Voltaire wanted universal tolerance so bad that he put all the arguments - philosophical, judicial/political, ethical, practical - he could find into the blender and came up with this treatise.
As with all of Voltaire's works (and the works of other contemporaries like Montesquieu and Diderot), this book can be read in a couple of hours. This will certainly not be a waste of time, considering the important message!