Hebrews – The Diagnosis
When we have become dull in hearing the Word, our ability to exercise discernment is diminished.
Warning Against Apostasy
11 About this we have much to say, and it is hard to explain, since you have become dull of hearing. 12 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, 13 for everyone who lives on milk is unskilled in the word of righteousness, since he is a child. 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.
6:1 Therefore let us leave the elementary doctrine of Christ and go on to maturity, not laying again a foundation of repentance from dead works and of faith toward God, 2 and of instruction about washings,  the laying on of hands, the resurrection of the dead, and eternal judgment. 3 And this we will do if God permits. 4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. 7 For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. 8 But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
9 Though we speak in this way, yet in your case, beloved, we feel sure of better things—things that belong to salvation. 10 For God is not unjust so as to overlook your work and the love that you have shown for his name in serving the saints, as you still do. 11 And we desire each one of you to show the same earnestness to have the full assurance of hope until the end, 12 so that you may not be sluggish, but imitators of those who through faith and patience inherit the promises. (ESV)
 6:2 Or baptisms (that is, cleansing rites)