The quotation is from "The Humanitarian Theory of Punishment", which can be found by itself online.

BTW, anyone searching out the book should beware that there are two versions, one a subset of the other and not including this essay. The shorter volume is "God in the Dock: Essays on Theology", which is the first section of the longer, "God in the Dock: Essays on Theology and Ethics", also published under the title "Undeceptions: Essays on Theology and Ethics".

The essay called "God in the Dock" actually has little connection with its title. It is about the difficulties he found presenting the Christian faith to modern (i.e. of 1948) unbelievers of the working classes, based on his experiences in teaching soldiers in the R.A.F. These difficulties are mainly about wide differences in cultural and intellectual background.

The closing sentences of the essay may have wider application:

The simple, emotional appeal ("Come to Jesus") is still often successful. But those who, like myself, lack the gift for making it, had better not attempt it.