As of today 27 Jan 21 until 01 Mar 21, Belgium has banned all non-essential travel to and from the country.
All personnel travelling to or through Belgium must carry a statement of honour declaring their trip as essential. This includes all daily commuters, etc.
The statement is available in French and Dutch by following this Link and gives a wide range of options of why you need to travel.  Microsoft Edge will also allow automatic translation to different languages if required.  
The Belgian police will be carrying out both mobile and static checks in order to enforce compliance.