In this part of the story, Tobit realizes that there is pertinent information that his son, Tobias, needs to have before he dies. Since he has asked God to put him out of his misery (3:6), he calls for his son. Tobias comes to his father and Tobit gives him instructions concerning family, practical moral living, and about obtaining the funds that are left in trust.
This reading is pretty basic and mostly an allusion to the Mosaic Law, though it does something different from what the Mosaic Law does. The purpose of the Mosaic Law was to stand as a testimony against the people of Israel and to “increase the trespass” (cf. Romans 5:20-21, Deuteronomy 31:26). It was to reveal that people are unable to live up to God’s glory. Tobit, here, cites it as a sort of material prosperity gospel. If Tobias could keep the Law and be righteous like his father, Tobit, he would prosper at home and in society even though the family had little. The family trust fund served as an example of this worldly prosperity in return for ones merit. Like we saw in chapter 1, verse 3, we see that there is a continued skewed view of righteousness in this story. Since this book influenced many of the Jews during the First Century AD, we see, again, why there will be a tendency toward legalism and toward working for some sort of material prosperity as we read through the Gospels. We also see why there will seem to be this doctrinal idea that if people keep the Law, they will prosper materially and remain healthy (cf. John 9:2). This type of doctrine is based on an uninformed understanding of the Mosaic Law. The lack of prosperity and good health is meant to reveal the sinfulness of the nation. These promises had little to do with personal living and were given with the foreknowledge and fore-revelation that people would not be able to meet the conditions of God’s Law (cf. Deuteronomy 31:27). These Biblical doctrines are entirely absent from the theology of Tobit.
This being the case, we must realize that God’s Law is still His standard for morality and practice. It has been revealed that we fall short of God’s glory. He saves by grace alone. Through the faith that He provides, He does bring His people into conformity to His Law by writing it upon their hearts. So, we do not read this and presume that the instructions are bad instructions. Instead, we recognize that the Law is good, but we are not righteous. Nor can we attain righteousness of ourselves by keeping the Law. God alone is righteous.
Another interesting note in these verses is that verse 15 is an allusion to Leviticus 19:18, only verse 15 is in the negative form. This negative form will used by the either the apostles or the apostolic fathers in Didache 2. When Jesus teaches this golden rule, He will quote the positive formulation found in Leviticus 19:18. Both the positive and negative linguistic forms mean the same thing. Some will claim that Jesus quotes the Apocrypha, here. This is not the case. Jesus quotes Leviticus. Tobit quotes the negative form of Leviticus. While Jesus does allude to the Apocryphal works in many of His teachings, primarily to correct the unsound doctrine that the Pharisees and scribes have gleaned from these works, He does not quote the Apocrypha in this instance.