In Shakespeare's Romeo and Juliet, in Act I, scene v, state the meaning of "gentle sin" as Romeo compares Juliet to a.........
In Act One, scene v, in Shakespeare's Romeo and Juliet, the two lovers are meeting for the first time at a Capulet party. Neither of them know that the other is a member of their family's enemy.
When they meet, Romeo and Juliet flirt first with words. He compares Juliet to a shrine, and his lips to pilgrims—travelers who visit holy shrines to give thanks and worship. (These are metaphors.)
If I profane with my unworthiest hand / This holy shrine, the gentle sin is this: / My lips, two blushing pilgrims, ready stand / To smooth that rough touch with a tender kiss. (I.v.98-101)
Romeo declares that Juliet is line a shrine—a holy place.
Dictionary.com defines a shrine as:
...any structure or place consecrated or devoted to some saint, holy person, or deity, as an altar, chapel, church, or temple.
The extended metaphor created here between the two as they continue to speak and even kiss, uses references with a religious motif. Once Romeo identifies the young Juliet as a shrine, he says that if he has offended her, he will, like "two blushing pilgrims" smooth his offense (his "gentle sin") with a kiss.
Juliet responds that he is too harsh regarding the sin of his hands, and continues the discussion with religious references such as "devotion" "holy palmer's kiss." Romeo goes on in kind with similar references such as "saint" and "prayer."
In all, Romeo is comparing Juliet to someone just this side of heaven, to be worshiped and adored, as pilgrims would worship a shrine.