Challenge: If God Requires Atonement, He Lacks Something

Here's a question I came across:

God is defined as an Almighty being. An Almighty being does not require atonement (for “sins”). Therefore if God requires atonement as the Bible says, he is imperfect and not Almighty.... In other words, philosophically, the need for atonement indicates a lack of something, which detracts from the perfection which God should have.

Can you clear this up for the questioner? Can you explain what atonement is, why it's necessary, and how it relates to God's perfection? This isn't an overly tricky one, but it does require both an understanding of the doctrine and an ability to explain it clearly and concisely. Give it a try in the comments below, then we'll hear from Brett on Thursday.

[Explore past challenges here and here.]