The final scene of Act II opens with "so smile the heavens upon this holy act...." Of what holy act is Friar speaking?What does this foreshadow as predicted by the Chorus in the Prologue to act I?
In the Prologue to Act I of Romeo and Juliet the Chorus states,
A pair of star=cross'd lovers take their life;
Who misadventured piteous overthrows
Do with their death bury their parents' strife. (1.1.6-7)
The marriage of Romeo to Juliet is the second of their "misadventured piteous overthrows"--a series of events that keep them apart. After the words of Friar Laurence,
these violent delights have violent ends,
And, in their triumph die, like fire and powder,
Which as they kiss, consume. (2.6.9-11)
Romeo commits his second act of "misadventured overthrow"--the first is invasion of the party and meeting with Juliet--as he encounters Tybalt and seeks to ameliorate the tense conflict between Tybalt and Mercutio. His well-meaning declaration of love for Tybalt now that he is related to the Capulets through marriage is misunderstood and backfires as it causes Tybalt to become so enraged that he stabs Mercutio under Romeo's arm.
As a result of his friend's angry death and curse upon "both your houses," Romeo loses his dear friend, whom he has tried to save from harm, Romeo's act of love for Tybalt becomes overthrown by the insidious act of Tybalt's having stabbed Mercutio and Romeo having, then, killed Tybalt--"a piteous overthrow," indeed.
Contextually, this quote is more important than just figuring out that the friar is referring to the wedding. The continuation of his words is "That after hours with sorrow chide us not!" (II.vi.2). The friar full well realizes that there could be severe consequences to the marriage. It is perhaps his greatest error in the play, next to leaving Juliet alone in the Capulet tomb at the end of Act V.
The friar's tone for Romeo, which is didactic and solemn, completely changes when Juliet enters the cell: "Here comes the lady, so light a foot...." (II.vi.16). He has been warning Romeo of the rashness of the decision, but will not allow himself to spoil the mood of Juliet or put doubt into her mind, though if either of the two characters could be convinced to not follow through, it would be she.
It is also important to note that neither the wedding nor and part of the the consummation of the marriage is shown on stage. What is Shakespeare's point?
This holy act to which the Friar is referring is likely the most important moment upon which the rest of the play hinges: Romeo and Juliet's wedding.
The friar is hopeful that this marriage will indeed be God's will and that it will change the situation between the two families. I think this quote you cite shows that the friar isn't sure if he's done the right thing but certainly hopes so. He took a great risk in not involving the parents in this scenario.
What is further foreshadows is these two star-crossed lovers deaths. Marriage is a vow of love that has now sealed for us that these are the lovers cited in the prologue of Act I.
What Friar Lawrence is talking about here is the wedding of Romeo and Juliet. This is, of course, a sacrament in the Catholic Church, and so it would be seen as a holy act. You can see that Friar Lawrence is talking about marrying the two of the from the last lines of the scene. The Friar says that the two of them cannot be together alone until the Church makes the two of them into one.
This brings us back to the part in the Prologue where it talks about two lovers being star crossed. Now we know for sure (as if it weren't already obvious) that these are the two lovers that are mentioned there.