In the passage quoted from Act II, Scene 6, Friar Lawrence has agreed to perform the marriage ceremony for Romeo and Juliet. Knowing that they might sin together if they are left alone, he feels it is better that they be married quickly.
In Act II Friar Lawrence has previously tried to reason with Romeo by explaining that acting on impulse is never wise:
These violent delights have violent endsAnd in their triumph die, like fire and powder,Which, as they kiss, consume. (2.6.9-11)
Juliet has been given time to go to church and Romeo has arranged to meet her at Friar Laurence's cell. When they arrive they cannot contain themselves and exchange a great deal of kissing and caressing. Friar Laurence tells them to follow him and he will not take very long in marrying them. He tells them that they will no longer beliving separate lives, but the church will join them as man and wife forever. They will become one in the eyes of God.
Here is a link to the part of the play where the line is said:
Because the two young people are so anxiously amorous toward each other, Friar Laurence is very anxious to geth them married for fear that they may do something sinful before they are married. "Come, come with me, and we'll do the job quickly. Because if you don't mind, I'm not leaving you two alone until you're united in marriage."
Friar Laurence speaks these words to Romeo and Juliet in Act II, scene 6. The young lovers have just arrived at the Friar's cell in order for him to marry them. He's just telling them to hurry in, and the vows they take will soon join them together as one.