Euler was a committed Christian and, apparently, a biblical literalist as well as being (arguably) one of the greatest mathematicians ever - he was certainly the most prolific (apart from perhaps Paul Erdos).
According to one website (condensed from E T Bell's Men of Mathematics):
Euler remained a Christian all of his life and often read to his family from the Bible. One story about his religion during his stay in Russia involved the atheistic philosopher Diderot. Diderot had been invited to the court by Catherine the Great, but then annoyed her by trying to convert everyone to atheism. Catherine asked Euler for help, and he informed Diderot, who was ignorant of mathematics, that he would present in court an algebraic proof of the existence of God, if Diderot wanted to hear it. Diderot was interested, and, according to De Morgan, Euler advanced toward Diderot, and said gravely, and in a tone of perfect conviction: "Sir, ( a + bn )/n = x , hence God exists; reply! " Diderot had no reply, and the court broke into laughter. Diderot immediately returned to France.Also on Euler:
- The works of Euler online
- a paper by Duncan roper on Euler and the Koinisberg bridges problem
- a brief bio
- Rouse Ball's bio of Euler
- Christian history Institute
- The Euler archive blog
- Eulogy of Euler by Nicolas Fuss:
He was entirely imbued with respect for religion and his piety was sincere and his devotion was full of fervor. He fulfilled with the greatest detail all the duties of a Christian. He loved everyone, and if he felt stirrings of indignations it was against those enemies of religion, especially against the declared apostles of atheism that he made a stand in the defense of the Revelation against the objections of atheists in a work which was published in Berlin in 1747.