This is part of a letter written by John Keats to his neighbour (and eventually his fiancee.) If you would like to read the letter in its entirety

I have been astonished that men could die martyrs for their religion –
I have shudder’d at it.
I shudder no more.
I could be martyr’d for my religion
Love is my religion
And I could die for that.
I could die for you.
~John Keats~