Why does Shakespeare use religious metaphors when Romeo and Juliet first speak?
Romeo and Juliet is a story of young love. Shakespeare uses religious imagery to reinforce the purity of their love and how they are drawn together like religious pilgrims to a holy site. He also wrote their conversation as a shared sonnet, which means that as Romeo speaks some lines, Juliet speaks echoing line.
Romeo refers to Juliet’s hand as a “holy shrine” and comments that he is to “profane” it; his lips are “two blushing pilgrims” drawn to her kiss. Juliet picks up on his religious imagery and makes witty jokes. As long as he has “mannerly devotion,” she is a saint that he, her pilgrim, may touch.
This conversation is composed as a shared sonnet, and sonnets are love poems; so the religious metaphor leads to perceiving their wordplay as the beginnings of pure love.
Then move not while my prayer's effect I take.
Thus from my lips, by thine my sin is purg'd. (Act 1, Scene 5)
This is also a witty play on words through which they are forgiven for transgressing with a kiss given in secret. Shakespeare wants to remind the audience that Romeo and Juliet are innocent youths who are experiencing a newborn, pure love.
In Act I, scene 5 of Romeo and Juliet, the lovers first meet use an extended metaphor to compare Romeo to a pilgrim who is approaching Juliet's sacred shrine. In line 105, Romeo even says that he is acting out a prayer when he kisses Juliet, and he says that "Now my sin has been taken from my lips by yours" (line 106). To reverse that breach of a holy shrine and return his sin to him, Juliet kisses Romeo again.
In one way, this scene can be read as Romeo and Juliet blaspheming religion. They are pretending to be saintly while engaging in forbidden love that their families don't know about. Part of why they use religious language might be to cover up their sins with a religious overlay. In other words, they know they are doing wrong, but are covering it up by pretending to be religious.
In a deeper way, however, Shakespeare is conveying that Romeo and Juliet's love is pure. While their relationship bucks the societal conventions of their day, they love each other, so their love could be seen as blessed by God in a meaningful way.
The formality of the sonnet form and of the religious metaphors connotes the sanctity and sense of purpose in Romeo's approach toward Juliet after he first sees her. The extended metaphor in the fourteen lines, of which each character speaks seven, is that of Romeo being a pilgrim and Juliet a saint. With the setting of the play in the fourteenth century, the time period in which the Canterbury Tales was written, pilgrimages to holy sites were well known travelers, so the metaphors are, indeed, relevant to their lives and timely.
Romeo is in such awe of Juliet's beauty and of the fact that she is unapproachable as a Capulet that he perceives her as one to whom he must not "profane [her] unworthiest hand" (1.5.94); he perceives her as one would a saint. Moreover, he is so touched by her beauty and his charged emotions that he feels moved spiritually as well as physically:
O, then, dear saint, let lips do what hands do.They pray; grant thou, lest faith turn to despair. (1.5.102-103)