What is Christian maturity with respect to sin and evil?

There is a practice called mithridatism. Are you familiar? It is the practice of ingesting small, non-lethal amounts of a poison in order to build up resistance or immunity to that poison. Is this what Christian maturity with respect to sin and evil is like? A mature Christian, then, is one who remains unaffected by sin and evil even when they directly or indirectly participate in it? John Owen helps us understand how to answer this question.

In the classic work The Mortification of Sin, John Owen points out that one of the great dangers of continuing in sin or, as Owen puts it, “harbouring your lust” is “Being hardened by the deceitfulness of sin.”[1] Owen has in mind the warning of Hebrews 3:12–13,

Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin (Hebrews 3:12–13).

What is the outcome of being hardened by the deceitfulness of sin? An evil unbelieving heart that leads one to fall away. This is the life cycle of sin. “Sin when it is fully grown brings forth death” (James 1:13–15). So continuing in sin ultimately leads to apostasy or effectively denying God’s offer of salvation in his Son. This should not surprise us. Sin, unrighteousness, by nature suppresses the truth in those who practice it (Romans 1:18). Therefore, if we continue to practice sin, refuse to repent, and, as a result, sear our conscience, then our powers of discernment become useless. We can no longer distinguish between good and evil effectively. Hebrews 5:14 calls this the opposite of Christian maturity,

But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil (Hebrews 5:14).

So Christian maturity with regard to sin and evil is in part being able to distinguish between good and evil. In a fallen world that often calls good evil and evil good, this is a vital skill. Another characteristic marks Christian maturity, though. Ironically, a certain immaturity marks Christian maturity.

In 1 Corinthians 14:20, Paul tells us that a necessary aspect of Christian maturity is immaturity with regard to evil. He writes,

Brothers, do not be children in your thinking. Be infants in evil, but in your thinking be mature (1 Corinthians 14:20).

Mature Christians think clearly with regard to good and evil. They are adults in their thinking not children. We’ve confirmed this above. However, Paul implies here that such maturity did not come by tasting evil and sin so as to build up an immunity to it. Paul says, be infants in evil. The full implication, then, is that such maturity came by being immature in the ways of sin and evil, that is, by not practicing it. Christians should not practice sin and evil even in the slightest. Paul makes this clear elsewhere in 1 Thessalonians 5:21–22,

but test everything; hold fast what is good. Abstain from every form of evil (1 Thessalonians 5:21–22).

Being able to distinguish between good and evil and being immature in evil, that is, abstaining from evil, while holding fast to what is good—these things mark Christian maturity.

Christian maturity with regard to sin and evil then is not like the practice of mithridatism. Notably, mithridatism will not work on certain toxins. As one continues to take small, non-lethal doses of certain toxins in the hopes of building immunity, they actually facilitate the opposite result. Rather than building resistance to the poison, the poison accumulates in the body until it reaches a lethal level, killing the practitioner of mithridatism. Sin is the poison we never build immunity to. We may consume it over and over in non-lethal doses with the result that we no longer feel its immediate effects. In doing so, we deceive ourselves into thinking we’ve built resistance to it—that we are now mature with regard to sin. But all the while the poison of sin has continued to accumulate and will continue to accumulate in our souls until it ultimately kills us. This is why John Owen wrote, “always be killing sin or it will be killing you.”[2]

Is Christian maturity as simple as that? To be sure there are other aspects that mark Christian maturity. But when our Lord spoke one of the most famous promises of his coming Christ through the prophet Isaiah, this is how he chose to describe him,

Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel. He shall eat curds and honey when he knows how to refuse the evil and choose the good (Isaiah 7:14–15).

If we can capture the idea of Christian maturity with the word Christlikeness, then this is at the essence of Christlikeness—holiness, turning from evil and choosing the good. When the Christian does this, they are mature. When the Christian does this, they are Christlike.

[1] John Owen, The Mortification of Sin (Carlisle, PA: Banner of Truth Trust, 2022), 68–69.

[2] Owen, The Mortification of Sin, 9.