"Train up a child in the way he should go: and when he is old, he will not depart from it. Proverbs 22:6"
A child is a very young human being who comes into the world naive and have very little knowledge about his/her environment. Children are eager to learn and quickly adapt to their sphere of contact or environment. It is for this reason that they need to be trained so that they can develop good morals and skills to be useful to the society when they mature.
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Since children are God's heritage He wants them to be trained in his Word because the way for them to go is right in the Word of God. The scriptures will teach them good and evil and how to choose good over evil.
The scriptures will also teach them how to make good decisions and good choices; in fact, the scriptures will direct them in the right path toward God and humanity. Parents who are the channel through whom the children came to the earth have a responsibility to ensure that the dream of God for these children become a reality.
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If you are a parent, know that God has commanded you to train your children in the way that he, God, wants them to go and when they grow into adulthood they will not depart from it.