Why are you a Theist?

[Angrboda]

Religion, after all, is considered by the common people to be true, by the wise as false, and by the rulers as useful. ~ Lucius Annaeus Seneca

As quoted in What Great Men Think About Religion (1945) by Ira D. Cardiff, p. 342; No original source for this has been found in the works of Seneca, or published translations. It is likely that the quote originates with Edward Gibbon who wrote:

The various modes of worship which prevailed in the Roman world were all considered by the people as equally true; by the philosopher as equally false; and by the magistrate as equally useful. --- Edward Gibbon, The History of the Decline and Fall of the Roman Empire, Vol. I, Ch. II

Elbert Hubbard would claim in 1904 (Little Journeys: To the homes of great philosophers: Seneca) that Gibbon was "making a free translation from Seneca".

When one is attempting to rebut an accusation that one's scholarship is inadequate, it's best not to resort to quotations of questionable authenticity, lest one be considered to have abetted that accusation.