SGANARELLE: But one has to believe in something; what is it that you believe? […]

DON JUAN: I believe that two and two are four, Sganarelle, and that four and four are eight.

[SGANARELLE: Mais encore faut-il croire en quelque chose dans le monde : qu’est-ce donc que vous croyez? […]

DON JUAN: Je crois que deux et deux sont quatre, Sganarelle, et que quatre et quatre sont huit.]

Molière (1622-1673) French playwright, actor [stage name for Jean-Baptiste Poquelin]

Don Juan [Dom Juan], Act 3, sc. 1 (1665) [tr. Wilbur (2001)]
    (Source)

This passage, where belief in folk spirits and bogeymen (or, alternately, math) is conflated with religious belief, was dropped from later performances, and is sometimes not included in text versions of the play (e.g., Clitandre (1672)).

(Source (French)). Other translations:

SGAN: People must believe something in this world. What do you believe? [...]
D JU: I believe that two and two are four, Sganarelle, and that twice four are eight.
[tr. Van Laun (1876)]

SGAN: One must believe in something here below. What do you believe in? [...]
JU: Well, I believe that two and two make four, Sganarelle, and that four and four make eight.
[tr. Wall (1879)]

SGAN: Now just tell me (for one must believe something) in what do you believe? [...]
D. JUAN: I believe two and two make four, Sganarelle, and that four and four are eight.
[tr. Waller (1904)]

SGANARELLE: But at least a man must believe in something here below. Now what do you believe in? [...]
DON JUAN: I believe that two and two make four, Sganarelle, and that twice four is eight.
[tr. Page (1908)]

SGANARELLE: A person must have faith in something. What do you believe? [...]
DON JUAN: I believe, Sganarelle, that two and two are four and four and four are eight.
[tr. Bermel (1987)]