Academic writing often involves collating information from a variety of sources into your own writing, to show you have researched widely and to add credibility to your work.
However, when using the work of others, you need to be careful not to plagiarise: stealing and passing off the work of someone else as your own.
To avoid doing so, make sure you properly acknowledge the author of the resource you have used, by quoting and paraphrasing.
e.g. The second Vatican Council was a significant event in Australian and international catholicism, which occured over 45 year ago. (Black, 1991, p. 20).
e.g. Lichtheim (1970) argues that anarchism as a social movement is now only significant in historical context, as the social implications are no longer relevant in the current world (p. 228).