Those who have been truly born-again (Jn 1:12-13; Jn 3:3-8; 1 Pe 1:23), who are indwelt by the “Spirit of Truth” (Jn 15:26; Jn 16:13; 1 Jn 4:6), who have been “enlightened” (Eph 1:18) via regeneration (new-birth), will grow in their ability to see the truth and the kind of lives we’re to live from God’s point of view. They will grow in their understanding of God Himself. They will move forward, not backward. True enlightenment does not lead one to reverse direction in their understanding of the truth. It will always lead one along a forward path of understanding and discernment. Truth leads to further truth, and light leads to further light. Like begets like. This describes the fruit of salvation. What we believe is the primary evidence of one’s salvation.
The Apostle Paul talks about this in Ephesians 1:17-20 and Colossians 1:9-10:
(Ephesians, 1:17-20) – 17 I pray that the God of our Lord Jesus Christ, the glorious Father, will give you spiritual wisdom and revelation in your growing knowledge of him, 18 – since the eyes of your heart have been enlightened – so that you can know what is the hope of his calling, what is the wealth of his glorious inheritance in the saints, 19 and what is the incomparable greatness of his power toward us who believe, as displayed in the exercise of his immense strength. 20 This power he exercised in Christ when he raised him from the dead and seated him at his right hand in the heavenly realms. (NET)
By God’s power we have been forgiven of our sins and regenerated. By God’s power we’ve been rescued from the penalty and power of sin. By God’s power we’ve died to sin and have been raised to new life in Christ. By God’s power we are kept secure in our salvation according to a faith that endures. By God’s power we’ve been given a new nature, a new heart—a spiritual capacity to know and walk with God, according to the will of God. Paul refers to spiritual enlightenment (vss. 17-18) in the form of prayer for the Ephesian Christians. As born-again believers ‒ who are indwelt by the “Spirit of Truth” ‒ we’ve been enlightened by the Holy Spirit to know the truth regarding Christ. This involves the Christian life and the doctrines of the Christian faith. In other words, those who are indwelt by the Spirit of Truth will not grow in error, but in truth.
Since our eyes have been opened to the truth, we’re enabled to learn and grow in our understanding of God, the Bible and the Christian life. If someone is regularly in God’s Word, yet not growing in understanding, there’s something seriously wrong. It indicates a lack of salvation. That’s especially evident if a person is actually losing ground as time goes on. If someone’s beliefs are actually becoming more confused and moving further away from the common understanding of historical, orthodox Christianity, then that reveals an absence of spiritual enlightenment. This indicates an absence of the new nature, an absence of the Spirit of Truth. Truth and light are progressive, not regressive.
As we “grow in the knowledge of God,” we’ll grow in “spiritual wisdom and revelation.” I like what the Life Application Bible Commentary says about this (bold mine):
The Holy Spirit gives “wisdom” (see also 1:8)—the ability to see life from God’s perspective, to have discernment. He also gives “revelation,” which refers to enlightened understanding, insight into God himself and the mysteries of divine truth.
Spiritual enlightenment regarding the truth, leads to an enlightened life regarding God and the teachings of the Christian faith, as well as a changed life that continues to grow in conformity to the kind of life that God has ordained for us as followers of Christ:
(Colossians 1:9-10) – 9 For this reason we also, from the day we heard about you, have not ceased praying for you and asking God to fill you with the knowledge of his will in all spiritual wisdom and understanding, 10 so that you may live worthily of the Lord and please him in all respects – bearing fruit in every good deed, growing in the knowledge of God, (NET)
True understanding of the truth, leads to a true understanding of the type of life that God has called us to. If Christians aren’t in God’s Word regularly, they will not grow spiritually. That’s one thing. However, if someone is in God’s Word regularly, yet their belief system becomes more and more confused and out of harmony with the common understanding of the Christian faith, then that’s reason for great concern. Again, it reveals an absence of spiritual enlightenment that comes via the new birth. Spiritual enlightenment changes our lives and brings us into conformity to “God’s will”—both doctrinally and in how we live our lives as followers of Christ.