"He is very easy to please, but very hard to satisfy."

 “Though Jesus Christ is very hard to satisfy, He is very easy to please. Think of that and it will help you a little. He is very easy to please, but very hard to satisfy. If you will but let Him in, and you have not much to put on the table. You cannot share much of life because you have not got it, He will be so pleased, if it be but a cup of cold water that you can give him. Let it be something genuine, something real.” – George MacDonald. From the sermon “The Father’s Appeal”, preached in Westminster Chapel.

Because of the frequent quotation by GK Chesterton and CS Lewis, the quote is usually rendered "God is easy to please but hard to satisfy," which is almost correct and does capture the sentiment exactly.