Isaiah 1:16

Though God had rejected their services as insufficient to atone for their sins while they persisted in them, yet he does not reject them as in a hopeless condition, but here calls upon them to forsake their sins, which hindered the acceptance of their services, and then all would be well. Let them not say that God picked quarrels with them; no, he proposes a method of reconciliation. Observe here, I. A call to repentance and reformation: "If you would have your sacrifices accepted, and your prayers answered, you must begin your work at the right end: Be converted to my law" (so the Chaldee begins this exhortation), "make conscience of second-table duties, else expect not to be accepted in the acts of your devotion." As justice and charity will never atone for atheism and profaneness, so prayers and sacrifices will never atone for fraud and oppression; for righteousness towards men is as much a branch of pure religion as religion towards God is a branch of universal righteousness. 1. They must cease to do evil, must do no more wrong, shed no more innocent blood. This is the meaning of washing themselves and making themselves clean, Isa 1:16. It is not only sorrowing for the sin they had committed, but breaking off the practice of it for the future, and mortifying all those vicious affections and dispositions which inclined them to it. Sin is defiling to the soul.

— Matthew Henry's Concise Commentary (public domain)

God saith to the sinner, "Wash you," &c., that he, finding his inability to "make" himself "clean," may cry to God, Wash me, cleanse me (Psa 51:2, Psa 51:7, Psa 51:10). before mine eyes--not mere outward reformation before man's eyes, who cannot, as God, see into the heart (Jer 32:19).

— Jamieson-Fausset-Brown Commentary (public domain)