The Presence of God is eternally holy! God chooses and elects those who serve Him in His court. The book of Holiness reveals the intricate details of the choosing of Aaron. The book of Exodus is a great book to study to understand the order of worship.
Those whom the Lord chooses are not necessarily the holiest of people, but He chooses them by His mercy, to showcase the example of His mercy, and to make an example of them.
God reveals to us the order of worship; He showed Israel the order of worship, in a way He did not reveal to Abraham, yet He revealed it to His descendants. In the book of Exodus, He gave Israel the ways of worship. It is therefore important that the worshipper understands the order of worship and stay reverent in God's Presence.
