--There is nothing we can do that will make God love us more. There's nothing we can do that will make God love us less.
Philip Yancey What's So Amazing About Grace?
"If Christianity does not make a man happy, it will not make him anything at all." William Barclay in his commentary on Philippians.
And I would agree, except that because I do not wish to confuse the two things, I would say "does not make a man joyous, it will not make him anything at all." Joy can bring happiness, but happiness is not the end of joy nor are they full equivalents. Happiness is part of joy--and joy when joy overflows, a kind of happiness accompanies it. But we shouldn't expect so much to be happy as to be joyous.