As most of the western european tribes, the Frisians worshipped Woden in early days.
The Frisians were a proud and free people. The Lex Frisonum is meantioned in some threads on this board, one of the first code of laws of the western European world that wrote down the rights of the individual Frisian. They were admired for this set of laws and it was said that every Frisian was a king.

So they valued Freedom a lot.

a) Which god(dess) gave them their freedom, and
b) When, and by who was this religion renounced.