But grow in the grace and knowledge of our Lord and Savior Jesus Christ. To him be glory both now and forever! Amen - 2 Peter 3:18
Different plants have different strategies for growth. Some are lean and tall; racing towards the sun. Others dig deep for water. Others spread broad leaves to catch each bit of sun that reaches them. Still others spread their roots wide. But all of them attempt to harness the resources of God to grow. That growth has a goal: maturity, the point at which a plant can begin to fruit and seed.
Peter reminds us to grow in both grace and knowledge. Paul describes this as a part of the development of every believer.
All over the world this gospel is bearing fruit and growing, just as it has been doing among you since the day you heard it and understood God’s grace in all its truth - Colossians 1:6
In both cases there is this idea that growth is spurred on by access to two things: the grace of God and the truth of God. You can speed up the acquisition of information, but the speed of spiritual development is not in your hands. In many cases, it requires that truth and grace be experienced through different seasons of life. Just as some plants are ready to fruit in mere days (like grasses), others may require several years. As Paul wisely noted elsewhere, it is God who causes the growth. Much of that is built into the spiritual DNA which differs for each one of us.
The Truth Of God
We must admit that there is and always will be a gap between our understanding and God's. We are always the learner; He is always the teacher. Growing in this area means learning, through experience, what we have already been taught. Peter is a good example of this. At one point in Jesus' ministry, he began to tell his students that he was going to get in trouble with the authorities and die. Peter didn't like this and "...took him aside and began to rebuke him. 'Never, Lord!' he said. 'This shall never happen to you!'" (Matt. 16:22)
Peter had the knowledge in his heart that Jesus was rabbi and that he was the disciple. But he also felt he knew what was going on, well enough to rebuke Jesus based his understanding of what was going on. In turn, Jesus uses the strongest language he will ever use on any disciple except Judas, saying, "Get behind me, Satan! You are a stumbling block to me; you do not have in mind the concerns of God, but merely human concerns" (vs. 23) Notice the gap both motive ("Satan") and the will of God ("concerns of God').
We want to take in copious quantities of the Word of God, from the Bible. Then we want the Spirit to take the Word of God through us, from life. Doing one without the other leaves us either shallow (from lack of the experience) or foolish (from lack of the Word). Truth guides growth, and love fuels it.
The Grace of God
We must come to grips with how much God loves us and has give us that we don't deserve. We don't have any claim on God's affections other than His own promises (because he cannot lie). If we are sure of this, then a great many mountains can be climbed and difficult tasks ventured, because his opinion is secure. A healthy believer sinks his or her roots into the love of God to fuel growth.
And I pray that you, being rooted and established in love, may have power, together with all the Lord’s holy people, to grasp how wide and long and high and deep is the love of Christ, and to know this love that surpasses knowledge—that you may be filled to the measure of all the fullness of God. - Ephesians 3:17b-19
Paul wants his "rooted" believers to take the next step: to discover the full extent of God's love and then experience that personally. Truth guides the growth, but love fuels it.
Growth Together
It is important to note that this growth is not designed to happen alone. Trees grow in groves. Grasses grow in fields and meadows. While each plant bears within it the full plan of God for the next generation, it does not mean that it was designed to do this alone, only that it can if absolutely necessary. We are better together with other believers. There is a nurturing environment created when those sharing the same spiritual DNA come together. That is why Paul says "together with all the Lord's holy people". It is a joint pilgrimage. A joint growth path.
Experience the truth by learning and teaching. You can learn from God. You can learn from others. But having to explain the truth yourself (to 'witness') from your own learning and experiences will cement it.
Experience grace by serving and being served. Peter had that there is a grace in both receiving service from others (John 13:8) and giving that service to others (John 13:15). Experience growth. Foster thriving relationships with others in the church.