This concept views the Christian life as a total and immediate change (2 Cor. 5:17; 1 John 3:19), rather than a gradual change which takes place through hard struggles. The idea is that what we need is one single act of commitment where I choose that Christ shall live and not I (Rom. 6:6; Luke 14:33). My life in Christ is the maintaining of that position (Gal. 5:16; Col. 2:6). This is what death to self means. I do not have the option of making any personal choices when I have made that choice that Christ shall live instead of me. So the yoke is “easy,” for sin is not difficult for Christ to overcome.
In Christ the nature becomes good immediately . It is Christ who lives and not I (Gal. 2:20). However, the person in Christ can only live the life of Christ in harmony with the knowledge which he has. Christ does not hijack a person's will or supernaturally give him perfect knowledge. We are still educated by the word of God. So while a person may have a perfect nature (mind - the mind of Christ), yet he may still be a babe in that he still has a lot to learn concerning God's will (1 Pet. 2:2). As a result he may even still be worshipping on Sunday for example. However, this is not counted as sin by God (John 9:41) because his nature is in perfect harmony with God's nature, even though he is not perfectly educated.
We may understand this concept better if we consider that when Jesus was a child of four, His obedience to God was perfect, but only insofar as He had knowledge of God's will (Luke 2:52). His nature desired only to obey, but He could only obey in harmony with the light which He had. When He was twenty His actions and His lifestyle were a more complete revelation of God's character than when He was four, but at every step, He was perfect because His nature was perfect.
(Source: Restoration Ministry)