The Alter Rebbe.
"Der Frumer vaAira" (5554). Long Version (Yeshiva, 5773/2013). The Alter Rebbe's Maamar about no sparks (or Jews) being lost, that it may (not only include but) necessitate Teshuva davka, and it cannot happen without this. Short overview (Beis Miodrash Linashim, 5773/2013). Review of "the Frumer VaAira" of the Alter Rebbe.