Much of what is known of Euler's religious beliefs can be deduced from his Letters to a German Princess and an earlier work, Rettung der Göttlichen Offenbahrung Gegen die Einwürfe der Freygeister (Defense of the Divine Revelation against the Objections of the Freethinkers).
Keyakinan agama Euler bisa dilihat dari suratnya kepada seorang Putri Jerman dan karyanya sebelumnya, Rettung der Göttlichen Offenbahrung Gegen die Einwürfe der Freygeister (Mempertahankan Wahyu Ilahi terhadap Keberatan Para Pemikir Bebas).