The original book is, in part, an attack on the notion of Freud and Feuerbach that religion is just projection or wishful thinking. Here’s a taste of Sproul’s book courtesy of Apologetics 315:
The Christian God has some ‘attractive’ features that might incline a person to embrace God as a narcotic to help him face the threatening character of life, but these are overwhelmingly outweighed by the trauma of encountering God. Though man may desire and create for himself a deity who meets his needs and provides him with innumerable benefits, he will not instinctively desire a God who is holy, omniscient, and sovereign.
The Bible does not try to conceal the fact that, in spite of God’s love and mercy, He is an awesome, threatening Being, a Being that man would not instinctively search for. The psychologists continue to argue that men like to invent protective deities that will provide them with comfort and security. But they cannot argue that men would invent the intimidating Holy One of Israel.
Sproul’s book also provides one possible explanation for atheism:
The New Testament maintains that unbelief is generated not so much by intellectual causes as by moral and psychological ones. The problem is not that there is insufficient evidence to convince rational beings that there is a God, but that rational beings have a natural hostility to the being of God.
Here’s a link to another expression of the same thought from Sproul’s web site.