Philippians 1:9-10. And this is my prayer: that your love may abound more and more in knowledge and depth of insight, so that you may be able to discern what is best and may be pure and blameless for the day of Christ,
1 Corinthians 14:20 Brothers, do not be children in your thinking. Be infants in evil, but in your thinking be mature.
Ephesians 5:17 Therefore do not be foolish, but understand what the will of the Lord is.
Colossians 1:9 And so, from the day we heard, we have not ceased to pray for you, asking that you may be filled with the knowledge of his will in all spiritual wisdom and understanding,
2 Peter 1:5-6 For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness,
2 Peter 3:18 But grow in the grace and knowledge of our Lord and Savior Jesus Christ.
Hebrews 5:14 But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil.
1 Thessalonians 5:21 but test everything; hold fast what is good.
So it is with the higher doctrines of Christianity. They can be understood and appreciated only by those who are advanced in Christian experience. The meaning is, that by long use and habit they had arrived to that state in which they could appreciate the more elevated doctrines of Christianity. Experience will thus enable one to discern what is suited to the soul of man; what elevates and purifies the affections, and what tends to draw the heart near to God. To discern both good and evil – That is, in doctrine. They will appreciate and understand what is true; they will reject what is false. (Barnes)
Discern both good and evil—as a child no longer an infant (Isa 7:16): so able to distinguish between sound and unsound doctrine. The mere child puts into its mouth things hurtful and things nutritious, without discrimination: but not so the adult. (Brown)
Even those who by reason of use; even those who by a gracious habit of wisdom and knowledge infused and perfected by long study, practice, and exercise of themselves in the word of righteousness, by which they are able to apprehend and improve the highest doctrines of the mystery of Christ. To discern both good and evil: for the discerning and differencing things, so as the mind discerns what doctrine is true and what is false by the word of righteousness, and the will chooseth what is good and refuseth what is evil
This does not intend a perfection of justification; for though some have a greater degree of faith than others, and a clearer discovery of their justification, yet babes in Christ are as perfectly justified as more grown and experienced believers; nor a perfection of sanctification, for there is no perfection of holiness but in Christ; and though the work of sanctification may be in greater perfection in one saint than in another, yet all are imperfect in this life; and as to a perfection of parts, babes have this as well as adult persons: but it designs a perfection of knowledge; for though none are entirely perfect, yet some have arrived to a greater degree of the knowledge of Gospel mysteries than others, and to these the strong meat of the Gospel belongs; they are capable of understanding the more mysterious parts of the Gospel; of searching into the deep things of God; and of receiving and digesting the more sublime truths. (Gill)