How can I know so surely that I'll love you

No matter what the future has in store?
Time is like a cave in which our torches
Show only the circumference of our minds. 
But love is will far more than it is passion,
Though passion may at first sustain the will.
One chooses love the way one chooses faith
Because that is the way that heaven lies.
My love for you is vaster than the ocean,
More rich in loveliness than coral seas.
I could no more relinquish it than let go
Willingly the precious gift of life.