Mrs. Chana Shukrun is a baalas Teshuvah and lives in Yerusholayim. A few years ago her brother who lives in France called her with the news that he is planning to get married, to a wonderful nice girl, but not Jewish.
Mrs. Shukrun was obviously horrified to hear this, and she tried to convince him out it, but to no avail, he is firm in his decision, and no one will budge him from it. Of course it upset him that his sister was so unhappy, but definitely not enough to convince him out of his decision. Eventually she convinced him to at least go to 770. “Just go there, for me, just for half an hour!”
Her brother laughed, but agreed to go, at least to appease his sister, letting her know that it should be clear that his visit to 770 will not change his decision. Mrs. Shukrun wasn’t fazed and said “you just go! For half an hour”
Indeed, he went to 770, before entering he set a stopwatch for half an hour so he could tell his sister that he was there for half an hour, and nothing happened. He walked inside, looked about here and there, and… as he expected, no electric spiritual lightening zapped him…
It’s already 27 minutes on his stopwatch, and he’s preparing to leave, a young French boy stops him and asks him where he’s from and if he would like a tour of 770. He showed him around, “this is where The Rebbe Davens, This is where the Rebbe farbrengs etc.” and he offered him to learn a maamer chassidus. Four hours later, he walks out of 770 and calls his sister. “Listen, this that I decided to break the engagement, I decided. The question is how to let her know without offending her”
He didn’t know why, but it was clear after being in 770 that a Jew cannot marry a non-Jew.