In one of your books you write that davening with Modern Orthodox people who aren’t serious about serving Hashem is compared to sha’atnez – a mixing that is unfavorable in the eyes of Hashem. But I thought that we are obligated to daven with all Jews – sinners as well?
No. We are not obligated to daven with all Jews. No. Because it is of utmost importance to be mi’kayeim the ideal of “harcheik mi’shachein rah.” Keep far away from a bad neighbor. Far away. And it is ossur to be mishtatef with a rasha even for a dvar mitzvah. All the poskim say that clearly. It’s ossur to join resha’im even for the purpose of doing a mitzvah.
Therefore it’s important not to be bribed by the fact that we are all davening the same words. You have to know that even in the best shul, if you sit down next to a person of bad character, he can ruin you chalilah. I’ve seen it again and again.
Once a man came into our shul. He was a b’al teshuva and full of enthusiasm. Full of eagerness to learn. And he sat down next to an old man who I knew was a leitz. And the old man kept on talking to him during davening. Speaking all the time against everybody in the shul – including me. Finally this b’al teshuva became so ruined, so spoiled, that he became my enemy and he left the shul. I wanted to make something out of him but this sho’chein rah ruined him. It happened twice in my history. Twice I had such an experience.
And therefore, it’s so important to avoid people who might spoil you. Even if it will only spoil you a little, you must beware. Even in the Yeshiva you must be careful. Every Yeshiva has an underworld. It doesn’t mean that it’s like the underworld in Manhattan but every Yeshiva has it’s underworld.
Keep far away from the underworld. Go to the upper world. It’s of utmost importance to seek out the best ones wherever you go.