“Reason, Observation and Experience — the Holy Trinity of Science — have taught us that happiness is the only good; that the time to be happy is now, and the way to be happy is to make others so. This is enough for us.”

"The Gods" (1876) as published in The Gods and Other Lectures (1879); this was one of his earliest versions of what became known as his "Creed". Some variants:
Justice is the only worship.
Love is the only priest.
Ignorance is the only slavery.
Happiness is the only good.
The time to be happy is now,
The place to be happy is here,
The way to be happy is to make others so.
Wisdom is the science of happiness.
As quoted in Familiar Quotations (1937) edited by Christopher Morley, p. 603
Happiness is the only good.
The place to be happy is here.
The time to be happy is now.
The way to be happy is to make others so.
Variant, as it appears on a manuscript copy he jotted down for a fan (26 March 1897) <!-- I know I have seen an image of this, and might be able to find a copy somewhere... ~ Kalki 2007·08·11 -->
Context: Reason, Observation and Experience — the Holy Trinity of Science — have taught us that happiness is the only good; that the time to be happy is now, and the way to be happy is to make others so. This is enough for us. In this belief we are content to live and die. If by any possibility the existence of a power superior to, and independent of, nature shall be demonstrated, there will then be time enough to kneel. Until then, let us stand erect.