You need milk, not solid food; for every one who lives on milk is unskilled in the work of righteousness, for he is a child. But solid food is for the mature, for those who have their faculties trained by practice to distinguish good from evil.

St. Paul's Letter to the Hebrews 5:11-14; 6:1-8

BRETHREN, about Melchizedek we have much to say which is hard to explain,
since you have become dull of hearing. For though by this time you
ought to be teachers, you need some one to teach you again the first
principles of God's word. You need milk, not solid food; for every one who
lives on milk is unskilled in the work of righteousness, for he is a
child. But solid food is for the mature, for those who have their
faculties trained by practice to distinguish good from evil. 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, with instruction about ablutions, the laying on of
hands, the resurrection of the dead, and eternal judgment. And this we
will do if God permits. For it is impossible to restore again to
repentance those who have once been enlightened, who have tasted the
heavenly gift, and have become partakers of the Holy Spirit, and have
tasted the goodness of the word of God and the powers of the age to
come, if they then commit apostasy, since they crucify the Son of God
on their own account and hold him up to contempt. For land which has
drunk the rain that often falls upon it, and brings forth vegetation
useful to those for whose sake it is cultivated, receives a blessing
from God. But if it bears thorns and thistles, it is worthless and
near to being cursed; its end is to be burned.