atonement will be made for you

This can be translated in active form. Alternate translation: "Aaron will make atonement for you" (See: rc://en/ta/man/jit/figs-activepassive)

to cleanse you ... so you will be clean

People who are acceptable for God's purposes are spoken of as if they were physically clean. (See: rc://en/ta/man/jit/figs-metaphor)