Deuteronomy 13 Test
The Deuteronomy 13 Test is one of several tests found in the Bible to distinguish between true and false teachers or prophets (as well as between true and false teachings). Deuteronomy 13 defines serving God as keeping his law, and defines idolatry as rebellion against or the rejection of his law. Typically, the Deuteronomy 13 Test refers to only one aspect of the test described in the chapter that it is named after, but there are multiple aspects to this test. The first aspect involves God testing us by allowing the prophecies, signs, and wonders of a false prophet or teacher to come true. The purpose of this test is to see if we will keep God's commandments or if we will follow the false prophet or teacher into rebellion against God's commandments. The second aspect of the test concerns whether the teacher or prophet openly leads people into idolatry. The third aspect of the Deuteronomy 13 Test (which is what is usually being referred to) checks to see if a teacher or prophet is teaching obedience to or rebellion against God's Law. If it's the latter, then the teacher or prophet fails the test and is therefore a false teacher.
All must be tested against Deuteronomy 13. This includes all who contributed to the content of the Bible, even Jesus himself and his disciples. A person is tested against Deuteronomy 13 after passing the Deuteronomy 18 Test and before being tested against any other Scripture. All teachings and interpretations of Scripture are tested against Deuteronomy 13 first.
"If a prophet, or one who foretells by dreams, appears among you and announces to you a sign or wonder, and if the sign or wonder spoken of takes place, and the prophet says, 'Let us follow other gods' (gods you have not known) 'and let us worship them,' you must not listen to the words of that prophet or dreamer. The Lord your God is testing you to find out whether you love him with all your heart and with all your soul. It is the Lord your God you must follow, and him you must revere. Keep his commands and obey him; serve him and hold fast to him. That prophet or dreamer must be put to death for inciting rebellion against the Lord your God, who brought you out of Egypt and redeemed you from the land of slavery. That prophet or dreamer tried to turn you from the way the Lord your God commanded you to follow. You must purge the evil from among you." [Deuteronomy 13:1-5; NIV]
"If your very own brother, or your son or daughter, or the wife you love, or your closest friend secretly entices you, saying, 'Let us go and worship other gods' (gods that neither you nor your ancestors have known, gods of the peoples around you, whether near or far, from one end of the land to the other), do not yield to them or listen to them. Show them no pity. Do not spare them or shield them. You must certainly put them to death. Your hand must be the first in putting them to death, and then the hands of all the people. Stone them to death, because they tried to turn you away from the Lord your God, who brought you out of Egypt, out of the land of slavery. Then all Israel will hear and be afraid, and no one among you will do such an evil thing again." [Deuteronomy 13:6-11; NIV]
The text presents only two possible options: Serve God and keep his Law, or follow other gods. In this way, God has defined rebellion against his commandments as serving other gods, so that one is only truly serving God if they are keeping his Law. Therefore, the one who rebels against God's Law does not love, follow, or serve God even if that person claims to do those things. Such a person serves other gods.
Deuteronomy 12:32 warns us not to add to or subtract from what God has commanded, but to do all he commanded. The entirety of the chapter that it is in warns us not to worship God the way that the pagans worship their gods. And that we are to worship God as he commanded us to worship him. It is also a warning against idolatry. It can be argued that adding to or subtracting from God's commands is also a form of idolatry, since the command against it is between two major passages warning against idolatry. Subtracting from God's commands is an obvious form or rebellion against God, as the person who subtracts from what God commanded has rejected what he or she has subtracted. And the one who calls something "sin" that is not contrary to any of God's commands is guilty of adding to God's commands.