The answer is Your beauty will never fade, because you will live forever in the lines of this poem.
Step-by-step explanation:
Sonnet 18 by Shakespeare is a love poem that focuses on the idea of eternity. Indeed, in this poem, Shakespeare explains certain elements such as the beauty of the woman he loves are eternal. This idea is expressed in "thy eternal summer shall not fade". Moreover, in the last verse, he explains his love and beauty of the woman are depicted in the lines of the poem, and because of this they will last forever "in eternal lines to time". Thus, the idea is "Your beauty will never fade, because you will live forever in the lines of this poem."