"For I have sworn before you"

[Albino]

Dear Teachers:

Yesterday I was listening to the recording of the John F. Kennedy inaugural speech. There you can listen to the famous passage: “it is not what your country can do for you, but what you can do for your country” . But my question is about the structure and the usage of the word “for” when he says "For I have sworn before you and Almighty God". I understand that it is very formal, but how is the pattern or the rule for use it in this way.

I hope to be clear with my doubt.

Best wishes.

Hector.

[svartnik]

for = because