John Chrysostom
Here he does not converse about Christ or other lofty subjects, because the minds he addresses are still tender. Therefore he also makes the exhortation brief, since children cannot follow a long discourse.
For the same reason he says nothing about the Kingdom of God. It is not for such an age to hear about this. Instead, he speaks of what the childlike soul most and especially wants to hear, namely, that it will live long.
If someone inquires why the Apostle does not speak to children about the Kingdom, but offers a commandment from the law of Moses, we shall say that he did this because they are still very young.
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He also did this because he understood very well that, if husband and wife are faithful to the law he laid out for them, it will not be very difficult for them to subject the children to the same law.
When a matter receives a good beginning and has a sure and firm foundation, it proceeds and is directed very easily. But to lay the foundation and set up the support is no small difficulty.