Between Man and His Fellow
A teaching ofRav Yisrael Lipkinהרב ישראל בן זאב וואלף ליפקין
Rav Yisrael stressed that the mitzvos between man and his fellow carry as much weight as those between man and Hashem. Keeping the ritual side of Yiddishkeit without working on one's relationships with others and with oneself, he held, was an unforgivable mockery.
Source: Teachings of Rav Yisrael Salanter