Spiritual growth normally takes time. Many of us hope if we read our Bible and do the right things, we will instantly and permanently change. When we come to faith in Christ, the guilt of sin is removed and we now have a relationship with God. Like an infant, we enter the process of growth. This is what Peter means by our growing in salvation (1 Peter 2:2)
In order for God to grow us into a mature person, reflecting His design for our life, our new knowledge of God (that comes from the Bible) must combine with our experience. We naturally resist change. After all we've lived life our own way for a long time. Simply learning the Truth is not enough for our faith to change our lives into a new way of living. We need help to continue in this new life. Involvement with other believers is an integral part of how we live.
Ephesians 4: 21-24 (The Message) says it clearly:
. . . My assumption is that you have paid careful attention to Him, been well instructed in the truth precisely as we have it in Jesus. Since, then, we do not have the excuse of ignorance, everything—and I do mean everything—connected with that old way of life has to go. It's rotten through and through. Get rid of it! And then take on an entirely new way of life—a God-fashioned life, a life renewed from the inside and working itself into your conduct as God accurately reproduces his character in you.