The most beautiful thing we can experience is the mysterious. It is the source of all true art and science. He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: his eyes are closed. This insight into the mystery of life, coupled though it may be with fear, has also given rise to religion. To know that what is impenetrable to us really exists, manifesting itself as the highest wisdom and the most radiant beauty which our dull faculties can comprehend only in their most primitive forms — this knowledge, this feeling, is at the center of true religiousness. In this sense, and in this sense only, I belong in the ranks of devoutly religious men.

Albert Einstein (1879-1955) German-American physicist
“What I Believe,” Forum and Century (Oct 1930)
    (Source)

Einstein crafted and recrafted his credo multiple times in this period, and specifics are often muddled by differing translations and by his reuse of certain phrases in later writing. The Forum and Century entry appears to be the earliest. Some important variants:

The most beautiful experience we can have is the mysterious. It is the fundamental emotion that stands at the cradle of true art and true science. Whoever does not know it and can no longer wonder, no longer marvel, is as good as dead, and his eyes are dimmed. It was the experience of mystery -- even if mixed with fear -- that engendered religion. A knowledge of the existence of something we cannot penetrate, our perceptions of the profoundest reason and the most radiant beauty, which only in their most primitive forms are accessible to our minds: it is this knowledge and this emotion that constitute true religiosity. In this sense, and only this sense, I am a deeply religious man.

— "The World As I See It [Mein Weltbild]" [tr. Bargmann (1954)]

The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. He who knows it not and can no longer wonder, no longer feel amazement, is as good as dead, a snuffed-out candle. It was the experience of mystery -- even if mixed with fear -- that engendered religion. A knowledge of the existence of something we cannot penetrate, of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms -- it is this knowledge and this emotion that constitute the truly religious attitude; in this sense, and in this alone, I am a deeply religious man.

— "The World As I See It [Mein Weltbild]" [tr. Harris (1934)]

The most beautiful and deepest experience a man can have is the sense of the mysterious. It is the underlying principle of religion as well as all serious endeavor in art and science. He who never had this experience seems to me, if not dead, then at least blind. To sense that behind anything that can be experienced there is a something that our mind cannot grasp and whose beauty and sublimity reaches us only indirectly and as a feeble reflection, this is religiousness. In this sense I am religious.

[Das Schönste und Tiefste, was der Mensch erleben kann, ist das Gefühl des Geheimnisvollen. Es liegt der Religion sowie allem tieferen Streben in Kunst und Wissenschaft zugrunde. Wer dies nicht erlebt hat, erscheint mir, wenn nicht wie ein Toter, so doch wie ein Blinder. Zu empfinden, dass hinter dem Erlebbaren ein für unseren Geist Unerreichbares verborgen sei, dessen Schönheit und Erhabenheit uns nur mittelbar und in schwachem Widerschein erreicht, das ist Religiosität. In diesem Sinne bin ich religiös.]

— Variant in "My Credo [Mein Glaubensbekenntnis]" (Aug 1932)

See parallel sentiments here, here, and here.