Increasing in spiritual maturity means becoming more like Jesus. So the question can be rephrased as "How can I become more like Jesus?" No one can become more like Jesus until that person is a Christian, so the journey to spiritual maturity begins with salvation. If you have not taken that first step, you can find out how at https://www.compellingtruth.org/become-a-Christian.html.
How can I increase in spiritual maturity?
The journey to spiritual maturity is one that God enables. We do have responsibility, but He is the one who makes us like Jesus. Philippians 2:12–13 shows both of these truths when it says, "Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure." Verse 12 says we are to "work out [our] own salvation." Practically, this looks like consistently seeking God, doing things like praying, reading the Bible, and fellowshipping with other believers. Philippians 2:13 says that "it is God who works in you." We can be sure that He will take us on the journey to spiritual maturity if we are committed to Him. It is the Holy Spirit who both gives us the desire to be more like Christ and who does the work of transformation in us. In 1 Thessalonians 5:23–24, Paul writes under the inspiration of the Holy Spirit, "Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ. He who calls you is faithful; he will surely do it." The previous verses (1 Thessalonians 5:12–22) are a series of admonitions on how to act in spiritual maturity, and then Paul finishes with the assurance that God "will surely do it." Since we are burdened with our sinful flesh as long as we live on this earth, our growth in spiritual maturity will be a never-ending journey. Paul expressed this dichotomy in Romans 7. In verse 23, he says, "I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members." But in verse 22, he says, "I delight in the law of God, in my inner being." He asks in verse 24, "Who will deliver me from this body of death?" and then answers his question in verse 25: "Thanks be to God through Jesus Christ our Lord!" So we have the assurance that God will develop spiritual maturity in us—we can't do it and don't have to do it.
At the moment of salvation, we are fully equipped with everything we need for spiritual maturity. We do not have to have a further experience, as some would say, in order to become like Christ. Second Peter 1:3 says that "His divine power has granted to us all things that pertain to life and godliness." Peter is addressing ordinary Christians, including those who may have just become Christians, not a set of "super-Christians" who have been blessed beyond salvation. Peter goes on in this passage to argue that "through [the promises of God] [we] may become partakers of the divine nature" (verse 4). He says that we should therefore "supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love" (1 Peter 1:5–7). And then in verse 10, Peter gives the result of increasing in these qualities, "for if you practice these qualities you will never fall." This is the essence of spiritual maturity. The beauty of this is that God will change us to be more spiritually mature. Our part is to want Him to and to allow Him to change us.
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