The relations of the soldier.

Here it says that it is just, because it is in accordance with the will of God. It is a righteous attitude because God has arranged it that way.

Source

The first lesson a soldier must learn is obedience to those who have authority over him. Because he has to obey orders. This basic preparation is learned at home. After the soldier has learned to obey, then he is in a position where he can be promoted to the rank of officer, in which he will give orders to others. Knowing how to give orders depends a lot on how the soldier has learned to obey. Basic training is received at home through the relationship between parents and children, and later in work relationships between managers and employees. The victories of the Christian life are won at home and in the workplace.

Remember that it was said that the Lord Jesus, as a son, went to Nazareth and was subjected to Joseph and Mary, his earthly parents.
There are two essential factors that should be considered in this verse and in this section:

1. Paul was supposed to be talking about the Christian home, a home like the one he had been referring to in chapter 5, dealing with the subject of the marriage relationship. The obedience of children to parents is limited to the sphere of "the Lord". Christian parents have the privilege of claiming their children for the Lord. We believe that we should all do it. Even when only one parent is a believer, he can claim his children for God. The apostle Paul said in First Corinthians 7:14, "Because the unbelieving husband has been sanctified by union with his wife, and the unbelieving wife has been sanctified by union with her believing husband." Of course, this does not mean that the child is a believer simply because he has a Christian father. It means that the father has the right to claim that child.

2. The word for "obey" here is different from the word found in 5:22. The wife has to submit. The wife occupies a place of equality with the husband, and submission is merely a matter of direction. Here the son has to obey, in the same sense that a servant has to do it; the same word was used in verse 5.
   Disobedience to parents is the last and lowest form of rebellion that will occur on earth. Paul said in Second Timothy 3: 1 and 2: "You must also know that dangerous times will come in the last days.2 Men will be lovers of themselves, greedy, vain, proud, blasphemers, disobedient to parents, unthankful, ungodly." So disobedience to parents will be one of the characteristics of the last days. Today we hear many cases in which children reject the authority of parents and there are cases of assault and even murder of parents. This is a sign that indicates the times in which we are living.

Of course there will come a time in the life of a young man in which he will begin to distance himself from his parents because he will have to become independent and when he gets married, start building his own home. God wants that from a certain age the youth begins to fend for himself. This effort to become independent is, of course, different from disobedience.

Credits for the author Luis de Miguel from the biblical school following the teacher.
Source