You were formerly darkness, but now you are light in the Lord; walk as children of light
Ephesians 5:8 describes the essential change of nature which occurs at salvation. It doesn't say you were in darkness; it says you were darkness. Darkness was your nature, your very essence, as an unbeliever. Nor does it say you are now in the light; it says you are light. God changed your basic nature from darkness to light. The issue in this passage is not improving your nature. Your new nature is already determined. The issue is learning to walk in harmony with your new nature.
Why do you need the nature of Christ within you? So you can be like Christ, not just act like Him. God has not given us the power to imitate Him. He has made us partakers of His nature so that we can actually be like Him. You don't become a Christian by acting like one. We are not on a performance basis with God. He doesn't say, "Here are My standards, now you measure up." He knows you can't solve the problem of an old sinful self by simply improving your behavior. He must change your nature, give you an entirely new self--the life of Christ in you--which is the grace you need to measure up to His standards.
That was the point of His message in the Sermon on the Mount: "Unless your righteousness surpasses that of the scribes and Pharisees, you shall not enter the kingdom of heaven" (Matthew 5:20). The scribes and Pharisees were the religious perfectionists of their day. They had external behavior down to a science, but their hearts were like the insides of a tomb: reeking of death. Jesus is only interested in creating new persons from the inside out by infusing in them a brand-new nature and creating in them a new self. Only after He changes your identity and makes you a partaker of His nature will you be able to change your behavior.
Prayer: Dear Jesus, thank You for rescuing me from the kingdom of darkness and transferring me into Your marvelous light. I choose to walk as a child of light today.