This statement is presented in quotes in The Philosophy of Religion and Advaita Vedanta (2008) by Arvind Sharma, p. 216, as a "Santayanan point", but earlier publications by the same author, such as in A Primal Perspective on the Philosophy of Religion‎ (2006), p. 161, state it to be a stance of Santayana without actually indicating or in any ways implying that it is a direct quotation.
Disputed

“Oblivious of Democritus, the unwilling materialists of our day have generally been awkwardly intellectual and quite incapable of laughter. If they have felt anything, they have felt melancholy. Their allegiance and affection were still fixed on those mythical sentimental worlds which they saw to be illusory. The mechanical world they believed in could not please them, in spite of its extent and fertility. Giving rhetorical vent to their spleen and prejudice, they exaggerated nature's meagreness and mathematical dryness. When their imagination was chilled they spoke of nature, most unwarrantably, as dead, and when their judgment was heated they took the next step and called it unreal.”

George Santayana

Source: The Life of Reason: The Phases of Human Progress (1905-1906), Vol. V, Reason in Science, Ch. 3 "Mechanism"

“Skepticism is the chastity of the intellect, and it is shameful to surrender it too soon or to the first comer: there is nobility in preserving it coolly and proudly through long youth, until at last, in the ripeness of instinct and discretion, it can be safely exchanged for fidelity and happiness.”

George Santayana book Scepticism and Animal Faith

The Works of George Santayana p. 65
Scepticism and Animal Faith (1923)

“The pint would call the quart a dualist, if you tried to pour the quart into him.”

George Santayana

Source: The Genteel Tradition in American Philosophy (1911), p. 60

“It is not politics that can bring true liberty to the soul; that must be achieved, if at all, by philosophy;”

George Santayana

"The Irony of Liberalism"
Soliloquies in England and Later Soliloquies (1922)

“Whenever a nation is converted to Christianity, its Christianity, in practice, must be largely converted to paganism.”

George Santayana

Source: Winds of Doctrine: Studies in Contemporary Opinion (1913), p. 36

“"There is no God, and Mary is his mother." Often, almost certainly incorrectly, attributed to Santayana himself. More plausibly attributed to Robert Lowell, as a sardonic description of Santayana's philosophy.”

George Santayana

Paul Mariani, "Lost Puritan: A Life of Robert Lowell" (1994), p. 159
Misattributed

“Friendship is almost always the union of a part of one mind with the part of another; people are friends in spots.”

George Santayana

"Friendships"
Soliloquies in England and Later Soliloquies (1922)

“[Everything] ideal has a natural basis and everything natural an ideal development.”

George Santayana

The Life of Reason: The Phases of Human Progress (1905-1906), Vol. I, Reason in Common Sense

“Experience has repeatedly confirmed that well-known maxim of Bacon's that "a little philosophy inclineth a man's mind to atheism, but depth in philosophy bringeth men's minds about to religion." At the same time, when Bacon penned that sage epigram… he forgot to add that the God to whom depth in philosophy brings back men's minds is far from being the same from whom a little philosophy estranges them.”

George Santayana

Source: The Life of Reason: The Phases of Human Progress (1905-1906), Vol. III, Reason in Religion, Ch. I

“Happiness is the only sanction of life; where happiness fails, existence remains a mad and lamentable experiment.”

George Santayana

https://owlquote.com/quotes/happiness-is-the-only-2jy3r26
The Life of Reason: The Phases of Human Progress (1905-1906), Vol. I, Reason in Common Sense