Yvonne van Duyn v. Home Office, Case C-41/74, CJEU, 4 December 1974

Restriction to the free movement of workers in the European Union on grounds of public policy. Belonging to the Church of Scientology. 

Art. 45 TFEU 
Art. 49 TFEU  
Art. 21 EUCFR  
Art. 10 EUCFR  
Directive 64/221/EEC, on the co-ordination of special measures concerning the movement and residence of foreign nationals which are justified on grounds of public policy, public security or public health 

1. A member State of the European Union, imposing restrictions to freedom of movement for worker justified on grounds of public policy, is entitled to take into account, as a matter of personal conduct of the individual concerned, which is at the basis of the restriction, the fact that the individual is associated with some body or organization, whose the activities are considered socially harmful, but which are not unlawful in that State. 

2. These provisions are lawful despite the fact that no restriction is placed upon nationals of the said member State who wish to take similar employment with the same bodies or organizations.
(In the instant case, the Home Office had refused to let a Dutch woman enter the United Kingdom, to take up employment as a secretary with the Church of Scientology – whose activities were considered to be socially harmful by the Government of the United Kingdom).