Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:
O no! it is an ever-fixèd mark
That looks on tempests and is never shaken;
It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come:
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
If this be error and upon me proved,
The sickle, often associated with the Grim Reaper, is a huge reaping instrument that cuts through crops the same way lives are harvested. It is not only bent itself, but can bend and distort whatever it encounters; including beauty and faith. 
 A compass, the mathematical tool, creates arcs that mimic the shape of the reaper’s sickle. Compass also means ‘reach’; the sickle is a large implement that catches everyone. A third meaning of compass, the navigational one, continues the extended  nautical metaphor  of earlier lines. 
 Note that this line has hard  alliterative  ‘ck’ and ‘c’ sounds, and hissing  sibilant  ’s' s. When spoken aloud it has a slow, clattering effect—as threatening as the meaning. 

Sonnet 116