Your question is missing the paragraph of "Preface to Buddhism and Buddhists in China" that will help answer it. I've found it online. The paragraph is as follows:
The purpose of each volume in this series is impressionistic rather than definitely educational. They are not textbooks for the formal study of Buddhism, but introductions to its study. They aim to kindle interest and to direct the activity of the awakened student along sound lines. For further study each volume amply provides through directions and literature in the appendices. It seeks to help the student to discriminate, to think in terms of a devotee of Buddhism when he compares that religion with Christianity. It assumes, however, that Christianity is the broader and deeper revelation of God and the world of today.
Answer:
The idea the author is trying to convey is:
That anyone can understand and enjoy the books.
Step-by-step explanation:
In the paragraph of "Preface to Buddhism and Buddhists in China", the author is trying to show his audience that the books they are about to read are not a dense type of literary work. The purpose of the books is not academical in the sense that they do not deepen the studies and explanations about Buddhism. They introduce them in a lighter manner, aiming to get readers interested in the subject rather than transforming them into scholars. They are not textbooks, but a less formally instructional reading that will help readers develop their knowledge and curiosity a little further. Therefore, anyone, scholars or non-scholars, would be able to appreciate and understand the books.