In Act II, scene iii, Romeo visits Friar Laurence to arrange his marriage with Juliet. Of course, being Romeo's confessor, Friar Laurence knows that Romeo has been, up to this moment, "in love" with Rosaline.
When Romeo shows up so early in the morning, the Friar's first thought is that Romeo has spent the night with Rosaline. He says, "God pardon sin. Wast thou with Rosaline?" And when Romeo explains that he has been "feasting with [his] enemy," and he has fallen in love with and wants to marry Juliet, the Friar is flabbergasted at his haste. He says:
Holy Saint Francis! What a change is here!
Is Rosaline, that thou didst love so dear,
So soon forsaken? Young men's love then lies
Not truly in their hearts but in their eyes.
Jesu Maria! What a deal of brine
Hath wash'd thy sallow cheeks for Rosaline.
. . .And art thou chang'd?
When Romeo says that Rosaline did not return his love, the Friar comments:
O she knew well
Thy love did read by rote that could not spell.
This means that Rosaline knows that Romeo is too fickle to really care deeply for a woman yet, he isn't far enough along in his "studies."
The Friar, despite his misgivings, decides to help Romeo and Juliet marry secretly, hoping it will mend the feud between their families.