The Friar marries both Romeo and Juliet without parental consent. This is a deception, but it is a lie of omission. Then, when he first becomes aware that Juliet's father has arranged a marriage to Paris, he holds his tongue. This is another lie of omission.
The first time we see him directly deceive Juliet's parents is after she "dies" when he chides them for their tears and tells them that she is in a better place now than she was when she was alive. There is some truth to this, in a twisted ironic way, but it is deception nonetheless, for he listens to their grief but does not tell them that she is alive.
He must also have deceived them in agreeing to marry Juliet to Paris, for when they discover Juliet, he has already arrived at the house for the wedding.
All of these lies are significant because he is a holy father; his job is to listen to the dark truths in people's hearts and to absolve them of their sins. Yet he does not reveal his own stains. Although there are two people (not including Peter and Romeo's "man") who are active in hiding the truth from the Capulets, the friar's deception seems the worst. Even though all along, he says he is doing it for a greater good (peace), to continue with this ruse seems an inconscionable act, especially since it comes from a "holy" man.