Tenet =  a principle, belief, or doctrine generally held to be true; one of the principles on which a belief or theory is based:

 
“Love is my religion — I could die for that — I could die for you. My Creed is Love and you are its only tenet.”
– John Keats

Art – “Lady in a Garden by Moonlight” by John Atkinson Grimshaw (1836 – 1893).