There is no verse in the Bible that says if you train up a child in the way of the Christian faith that he will never depart from it. However, there is a verse in Proverbs that is commonly used for this belief.
Proverbs 22:6, "Train up a child in the way he should go, even when he is old he will not depart from it."
Notice that the verse is not saying if you train a child in the Christian faith or even the Old Testament faith, that he will not depart from it when he is older. It says that as you train a child how he is to behave and what he is to do, that is what he will stick with later in life. This proverb is a general statement about the importance of raising our children properly because that is what they will retain later in life. The admonition of Proverbs is that early training of the child helps to set his path as an adult. Of course, this is all the more reason for us to train our children in the truths of the Christian faith and pray that the Lord works in their lives so that they will be dedicated to Christ.
The word "train" is חָנַךְ [chanak /khaw·nak/] The "AV translates as “dedicate” four times, and “train up” once. 1 to train, dedicate, inaugurate. 1A (Qal). 1A1 to train, train up. 1A2 to dedicate."1 So, we can see that we are to dedicate our children to truth and to train them, not force them, in the ways of the Lord.
However, there is no guarantee that our children will "remain" Christians in their adulthood. They are their own individuals with different personality traits, life experiences, and influences from others (Proverbs 1:8-19). We cannot control them. We cannot force them to be Christians. But, we can influence them greatly. As Christians, we should raise our children in a godly way, not exasperating them, but instead live the Christian life with them, teaching them the truths of who Jesus is. But as I already said, there is no guarantee that our actions will automatically result in our children maintaining their Christian faith throughout their lives.
In addition, there are other verses in the Bible that speak of training members of your household when they are young. Please consider the following.
- Genesis 18:19, "For I have chosen him, so that he may command his children and his household after him to keep the way of the LORD by doing righteousness and justice, so that the LORD may bring upon Abraham what He has spoken about him.”"
- Deuteronomy 4:9, "Only give heed to yourself and keep your soul diligently, so that you do not forget the things which your eyes have seen and they do not depart from your heart all the days of your life; but make them known to your sons and your grandsons."
- Ephesians 6:4, "Fathers, do not provoke your children to anger, but bring them up in the discipline and instruction of the Lord."
- 1. Strong, James. Enhanced Strong’s Lexicon. Woodside Bible Fellowship, 1995.