Al Mizan- Vol 32 Chapter 32-34Part of verse 33: 33 speaks of removing any impurity in any shape or form that might misguide a person from true beliefs or correct acts. Therefore, what naturally follows from this is divinely granted protection from error or sin, which is a mental faculty that Protects a person from false beliefs or ugly deeds. Therefore, "removing impurity" and "purification" must be understood with the Prophet's sinlessness in mind.