Unlike physical maturity, spiritual maturity is not something that develops with age. Unless a person has some kind of medical issue, they will naturally grow and develop over time. Spiritual maturity is quite different. It is not tied to age, intelligence, ability, or emotional development. I’ve known young people who are very mature and wise beyond their years, and I’ve known some older people who make very irresponsible decisions and who should’ve known better. 

Generally speaking, a mature person knows and can teach effectively from the word of God. The writer of Hebrews gives us a great example of this (Hebrews 5:12-14): 

For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food, for everyone who lives on milk is unskilled in the word of righteousness, since he is a child. But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil.

This verse tells us two things. First, we must develop the ability to articulate the fundamental truths of the gospel message. The Hebrews writer says that his readers have to be reminded of basic principles (Paul tells the Corinthians the same thing in 1 Corinthians 3:1-2). He says that they are unskilled in the word of righteousness. Why? Because Scripture has to be used, and that takes practice—both for our benefit and for others. 

Jesus says that we are to take the gospel into all the world and make disciples of all nations – we have to be able to use God’s word to do that. To persuade others, train them, and help them become Christ-followers. The believers to whom the book of Hebrews was addressed couldn’t do this. Essentially, the author says, “You’re a bunch of children.”

Second, we have to have our powers of discernment sharply honed. We must cultivate the ability to quickly determine the difference between right and wrong. This takes training or “constant practice.” The writer uses the word gymnazō, from which we get the English term “gymnasium.” It means to train, like an athlete who dedicates his or her time to training for a sport. That is how the spiritually mature study the word of God. It gives us the facility to be able to use Scripture well. But athletes can quit training, and what happens? They get slower, weaker, flabbier. The same thing happens to us spiritually if we quit training with Scripture. 

Spiritual maturity takes time, effort, and dedication. It isn’t easy, and there aren’t any shortcuts. Above all else, it takes a selfless drive to be relentlessly molded and shaped by God’s Word.