There was a chassid in Kotzk that was accustomed to shout in prayer “Merciful Father, merciful Father.”
Another Chossid who heard him laughed at him saying: “Who says he is your father? For, when a Jew sins, they are referred to as slaves and not children (Talmud, Kiddushin 36a).”
When Reb Mendel heard this he said: “Continue screaming father, until he truly becomes your father.”