Every sin is theft from Hashem

אא"ז מו"ר ז"ל אמר עמ"ש מצות תשובה והתודו בגזילה.

My grandfather, my teacher and master of blessed memory, said regarding the mitzvah of teshuvah (repentance), that the Torah expresses viduy (confession) specifically in the context of theft.

The Sefas Emes cites his grandfather (the Chiddushei HaRim), who noticed that the Torah frames the obligation to confess one's sins around the case of robbery — and he asks why theft is taken as the paradigm for all repentance.

כי כל עבירה בכלל גזל.

Because every aveirah (sin) is included in the category of theft.

Every sin is essentially an act of robbery: a person takes the vitality and resources that Hashem placed in the world and uses them against the ratzon Hashem (Hashem's will), as though stealing what truly belongs to Him.

שמי שמחזיר כל הדברים להשי"ת שהוא המחי' הכל והוא בעלים של כל הנבראים ומי שיודע זה אינו חוטא בודאי:

For one who returns all things to Hashem — recognizing that He is the One who gives life to everything and is the true Owner of all created things — such a person who truly knows this will certainly not sin.

Teshuvah is fundamentally "returning the stolen object": giving everything back to Hashem by acknowledging that He alone is the Source of all vitality and the Owner of all creation. Someone who lives with this awareness has no opening for sin, since he never imagines that anything is "his" to misuse.

Summary: Confession is bound up with theft because every sin is a form of robbing Hashem of what is His; true teshuvah means returning everything to Him by recognizing that He is the Source of all life and the Owner of all creation — an awareness that itself guards a person from sin.