This quote can be found in chapter 28 of The Screwtape Letters. The longer passage reads:
Prosperity knits a man to the World. He feels that he is "finding his place in it", while really it is finding its place in him. His increasing reputation, his widening circle of acquaintances, his sense of importance, the growing pressure of absorbing and agreeable work, build up in him a sense of being really at home in earth, which is just what we want.
Ultimately, to understand this quote, and to understand the larger letters and the ideas which shape Screwtape's thinking, you must remember that The Screwtape Letters is a Christian satire, based in a Christian vision of the universe and humankind in it.
Christianity, at its core, prioritizes spiritual concerns and spiritual life over worldly success and prosperity. Indeed, this vision tends to be central to Lewis's own thoughts, given his conviction that faith and devotion to God hold a transformative effect on the soul, such that these qualities bring out the best and most ennobling qualities of any given human being. Likewise, to turn away from that possibility and devote oneself towards worldliness and material existence tends to have very much the opposite effect (and this sense of self-destruction and deterioration is central to Lewis's conception of what damnation entailed).
These ideas prove the foundation of Screwtape's advice in Letter 28, where he opens by chastising Wormwood for taking such glee in the violence and destructiveness of war, even though this destructiveness defeats the very purpose of their designs to begin with. As Screwtape insists, the demons want human beings to live as long as possible so they can continue to fall under the potential influence of corruption, prizing worldly concerns above the spiritual, much to their own ultimate detriment (as far as Lewis would opine).