Question: What benefits do we receive from knowing and believing that the Father of our Lord Jesus Christ, His Son, and the Holy Spirit alone are the true God and that there is no other?
First, this benefit: since it is our highest good to know God aright, and since our bodies and souls were created and redeemed at a price so that they might be temples of God and He might be glorified in us, it is only through the knowledge of the one true God — Father, Son, and Holy Spirit — that we know that this takes place in us. As Christ said, “I will send you the Comforter” (john 16:7), and “The Father and I will come and make our home with him” (John 14:23). See also 1 Corinthians 6:19, 20
19 Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? 20 For you have been bought with a price: therefore glorify God in your body.
Second, it has the benefit that when we pray, we know and think about which God we are calling upon — the true God, who is truthful in His promises, who alone can help inasmuch as He alone is an almighty God, and who alone will help inasmuch as He is a truthful and faithful Father.
The heathen, on the other hand, worship gods who are not really gods but inventions of the Father of lies, the Devil. They neither can help, because they are not God, nor will help, because they are inventions of the Devil, who is a liar and a murderer.
Third, it follows from this that unless we wish to apostatize from the highest good and forfeit the true invocation of God, we cannot also serve strange gods, either inwardly or outwardly. Rather we must separate our faith, prayer, and confession from the prayer and false worship of the Turks and other heathen, who do not worship the true God, as God bids in 2 Corinthians 6:14-18.
14 Do not be bound together with unbelievers; for what partnership have righteousness and lawlessness, or what fellowship has light with darkness? 15 Or what harmony has Christ with Belial, or what has a believer in common with an unbeliever? 16 Or what agreement has the temple of God with idols? For we are the temple of the living God; just as God said,
“I will dwell in them and walk among them;
And I will be their God, and they shall be My people.
17 “Therefore, come out from their midst and be separate,” says the Lord.
“And do not touch what is unclean;
And I will welcome you.
18 “And I will be a father to you,
And you shall be sons and daughters to Me,”
Says the Lord Almighty.
That great danger exists if people do not separate themselves from all superstition but direct their faith and invocation toward something else (as in Popery they are directed toward the saints and the bread or Host that is worshiped). The Holy Spirit warns us of this danger through St. Paul’s words in 1 Corinthians 10:19-22.
19 What do I mean then? That a thing sacrificed to idols is anything, or that an idol is anything? 20 No, but I say that the things which the Gentiles sacrifice, they sacrifice to demons and not to God; and I do not want you to become sharers in demons. 21 You cannot drink the cup of the Lord and the cup of demons; you cannot partake of the table of the Lord and the table of demons. 22 Or do we provoke the Lord to jealousy? We are not stronger than He, are we?
If someone had asked an idolater whether he sacrificed to demons, he would have said “By no means; I sacrifice to God!” But the Holy Spirit says in this passage that what they sacrifice to idols they sacrifice not to God but to demons. For what is not according to the Christian faith is invented by the Devil and does not come from God. With such mendacious idolatry, therefore, one does not serve God but the Devil, the father of lies.
Caspar Olevianus, A Firm Foundation, p. 15