When Belgium was created in 1831, it had a constitution very similar to the French one which was issued after the French 1792 revolution. Its Constitution was in turn largely inspired by the US Constitution.
We have always been under freedom of religion. But it is true that Catholics would for instance give bread more easily on sundays to those who went to church than to those who didn't (between WW1 and WW2). These are minute details compared to what's going on in other parts of the world. But its true there was a persecution of the jews between 1933 and 1944.
This was a time when things went horribly wrong, of course.
Freedom of religion was an idea that started way back in the 16th Century with Henry IV and his Edict of Nantes. However, Louis XIV revoked it, which cause massive emigrations.

Raoul