Why Does the God of Romans 1 have to be Trinitarian?

An e-mail responder asked:

Can you tell me how should I then respond say for example to my biologically older brother who may use the Fristian approach to my defense of the christian faith?  How do I respond to him, what is the way to go about doing it?

The "Fristian objection" occurs when a detractor says, "Why do I need to appeal to the Trinity to make sense out of reality?  Why not a god that has four members in the godhead instead of three?  Why does it have to be a Trinity instead of say, a 'quadrinity'"?  A brief answer follows below:

1. We know that God can't be four-in-one because Scripture says so.  Either the all-knowing God of Scripture tells us who He is in such a way that we can know enough about Him to speak accurately about His nature or we're left to guessing.  The problem with natural revelation is that it doesn't give you specific information about God other than the fact that He's powerful, absolute, and personal.  This is why God has given us Scripture, which is also known as special revelation.  Scripture tells us that "In Him we live, and move, and exist" (Acts 17:28).  The "Him" in that verse has revealed some things about His character and nature in Scripture such that we can know them for certain, and one of those things is His Trinitarian nature.  The Father is God (Eph. 1:3); Jesus is God (John 20:28), and the Holy Spirit is God (2 Cor. 3:17).  We do not defend just any old god. Our defense is to stand up for the God of Scripture because we are commanded to do so (2 Cor. 10:4-5).
2. If the detractor is an atheist make sure that you explain to him that he just gave up his atheism in order to defend his atheism.
3. We are defending the God that is revealed in Romans chapter 1 and the rest of Scripture. In other words, Paul does not say that creation and conscience can reveal to us that God is explicitly trinitarian.  How would this be possible?  What Romans one tells us is that the creation reveals to us that God is the powerful, personal creator. We also know that we are morally accountable to our Creator. Therefore, our argument is not that the creation explicitly reveals a Trinitarian God, but that it reveals that God is indeed the creator, is powerful, and holds us morally responsible for our actions.  When someone brings up the Fristian objection, you can explain that God has not only revealed himself in creation, but has also revealed himself in Scripture and that the only way we're going to know anything specific about God's nature beyond what He's revealed to us in creation is if He Himself reveals it to us, and He has done this in Scripture.  When he says," How do you know that the God of Scripture is the true God and that some other god does not provide the preconditions for the intelligibility of reality?" it is then that you can respond, "Because there is no other god that has been revealed to us other than the God that is in Scripture and then upon philosophical, evidential, and historical examination, all existing worldviews fail to provide what's needed to make sense out of reality."

Thus, the only way you are going to know whether God is four persons in one being or three persons one being is if God himself reveals that to you such that you can know it for certain. This specific revelation has been given to us in the Bible.

IN CONCLUSION, how do we know that the Trinitarian God is the one true God versus some god that we haven't heard about yet? The answer is simple, the god of Romans one has revealed himself to us in Scripture, and that God is a Trinitarian God. Again, when the "Fristian objection" comes up, the appeal must be made to Scripture and not to philosophical arguments to try to prove the Trinitarian nature of the one true God.  If he argues that this is special pleading, you can say in response," No, this isn't special pleading, it's special revelation."