Before he left this world, Reb Shlomo of Karlin told his disciples that after his passing they should visit Reb Mordechai of Neshchiz and accept him as their Rebbe.
Reb Uri (the Seraph) of Strelisk* arrived soon after in Karlin and learning of the passing of the tzaddik. He asked his colleagues what instructions he had left for them. He then set out by foot on the long trek to Neshchiz.
Of all the many guests who were standing about in the Rebbbe’s antechamber there, waiting to receive his blessing, the individual who caught Reb Uri’s eye was a certain prosperous merchant who had come to consult the Rebbe on some forthcoming transaction and to request his blessing for its success. Reb Mordecai soon came out and greeted this individual warmly, as was his wont with all who came to him. In this direction, of true love of a fellow, his Divine service was in fact outstanding. But Reb Uri as he looked upon the face of this merchant could tell that only recently he had sinned seriously. He was incensed why should the tzaddik of Neshchiz extend such a cordial welcome to such a sinner?
Quick to read his thoughts, Reb Mordechai walked over to Reb Uri:
“Please leave this place right away. What do you want here in my room?” he said.
Weary, distressed and disappointed, Reb Uri found his way to the local beis midrash. But no sooner had Reb Mordechai concluded his conversation with the merchant than he went out to seek his visitor from Strelisk. When he found him, he said:
“What you knew, brother, I also knew – but do you know why your Rebbe of Karlin sent you here? In order to learn that the man in whom the love of a fellow Jew is not so entrenched in his heart that he is unable to love and embrace even a grievous sinner – such a man has not accomplished even half of his Divinely appointed lifelong task. For if you bring such a sinner close to your heart, without a doubt he will return to G-d with all his heart and being a baal teshuvah, a sincere penitent, he will rank higher than a perfect tzaddik – that is unless you know him to be one who transgressed willfully.”
Reb Uri stood silent: he had found a Rebbe from whom he would have much to learn in the years to come.
From A Treasury of Chassidic Tales by Uri Kaploun pgs 352-353
*Reb Uri of Strelisk 1757 – 1826