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Hebrews 6 (KJV)
6 Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God

Therefore Since, as was stated in the previous chapter, you ought to be capable of comprehending the higher doctrines of religion; since those doctrines are adapted to those who have been for a considerable time professors of Christianity, and have had opportunities of growing in knowledge and grace as much as strong meat is for those of mature years leave now the elements of Christian doctrine, and go on to understand its higher mysteries.” The idea is, that to those who had so long been acquainted with the way of salvation, the elements of Christianity were no more adapted than milk was for grown persons.