Common knowledge

Whenever you are discussing facts and opinions that can be considered general knowledge in your field, it is not necessary to cite sources.
The following statements are examples of this:

"At the beginning of the 20th century, Dutch society was strongly divided into ideological or faith-based 'pillars’."

"During the past years, the role of social media has become very important in our discipline."

Statement like these do not need to have a source cited.