Let me start by saying that it’s not wrong for a new believer to be immature any more than it’s wrong for a child to be childish.
Puerility is only annoying in an adult. When a four year old dons a cape and wears his underwear over his pants, claiming x-ray vision it’s cute. When his dad does that it’s concerning (or certifiable).
When you’ve been a believer for many years though, lack of these indicators should be concerning.
Mature believers possess these 5 indicators…
1. An Appetite for Meat
It’s good to enjoy the milk of the gospel with every meal. But some Christians pride themselves on focusing only on the gospel, snubbing the offer of deeper doctrines. The love of doctrine may need to be acquired over time but it will always be there in a mature believer.
The writer to the Hebrews chides his readers for their reluctance to chew.
Heb 5: 11-14 [W]e have much to say, and it is hard to explain, since you have become dull of hearing. 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.
An infant’s meal needs to go through a blender for the first few months of his or her life. When a normal 21 year old still asks mommy to spoon feed him mashed potatoes it is just creepy and dysfunctional.
2. An Imperviousness to Personal Offence
It is seldom that a mature believer feels offended. Offence is appropriate at any attack on God’s glory, as when the zeal of God’s house consumed Jesus and he aimed an Indiana Jones whip at the overpriced animals in the Temple’s corrupt commercial zone.
But mature believers don’t take personal offence easily. They understand that when someone sins against them there are bigger issues at stake than their personal rights; e.g. God’s glory, the attacker’s relationship with God, etc.
Take Paul. When he could no longer draw a crowd (being in jail for the gospel and all) rival preachers were pouring salt on his shackle-blisters by preaching the gospel in competition with him. He didn’t get uppity. Instead he seemed buoyed by the news that the gospel was still getting airtime. That’s maturity!
Phil 1: 15-18 Some indeed preach Christ from envy and rivalry, but others from good will. The latter do it out of love, knowing that I am put here for the defence of the gospel. The former proclaim Christ out of rivalry, not sincerely but thinking to afflict me in my imprisonment. What then? [So what?] Only that in every way, whether in pretense or in truth, Christ is proclaimed, and in that I rejoice. Yes, and I will rejoice.