At the beginning of Romeo and Juliet, Romeo is distraught by the ending of his relationship with Rosaline; however, once he meets Juliet, Rosaline is completely forgotten. At their first meeting, he makes the comparison, or metaphor, of Juliet to a holy shrine; his "pilgrim" lips stand ready to smooth the rough touch of his hand. Pilgrims are typically religious travelers who go long distances to visit a place of religious importance. Juliet continues this metaphor when she challenges his request of a kiss, stating "saints have hands that pilgrims' hands do touch, And palm to palm is holy palmers' kiss," essentially playfully suggesting they shake hands instead of kiss.
Romeo then compares her to a saint, another metaphor, and compares her kiss to a prayer.
ROMEO O, then, dear saint, let lips do what hands do; They pray, grant thou, lest faith turn to despair.
JULIET Saints do not move, though grant for prayers' sake.
ROMEO Then move not, while my prayer's effect I take. Thus from my lips, by yours, my sin is purged.
JULIET Then have my lips the sin that they have took.
ROMEO Sin from thy lips? O trespass sweetly urged! Give me my sin again.
Juliet continues the metaphor when she asks if her lips have taken his sin, and Romeo kisses her again when he says, "Give me my sin again."
Overall, all of the religious imagery and metaphors indicate their love is a spiritual one. He compares his longing for her as one of necessity, of passion, and with the same kind of goodness associated with God. It sets their relationship above the others mentioned in the text, specifically the one Romeo had with Rosaline.