Do you have to believe that Jesus is the 2nd Person to be saved?

So you're saying that the doxology of 2 Cor 13:14 is not referring to the Trinity?
The consensus of every theologian in history is the same argument the Catholics use for their doctrines and practices that the Protestants, or Low Protestants, at least, do not agree with. That was the universal consensus before the Reformation, except for a few small groups, who were universally rejected as heretical. And people did question it, but then were not considered "orthodox" based on that; just like those small groups who differed from the other Catholic doctrines.

Yes, the scriptures acknowledge Father, Son and Spirit, and often in that order, but it was not made into such a precise formula until the fourth century, and that to confront the heretical views. Then, the scriptures and earlier fathers were interpreted in light of the later theological development. You can even see this stuff admitted in places like Kelley's Early Christian Doctrine, Harper's Bible dictionary, etc.

 

The trinity is demonstrated in the Scriptures in Paul's writings in verses such as 2Cor.13:14, the baptism of Jesus, and many other Scriptures.

Its precise formula is commanded by Jesus in Mat.28:19,20, when Christ commands us to go; disciple, and baptize in the name of the Father, Son, and of the Holy Ghost. We are not to vary from that order, from that formula. That is the command that Jesus Himself gave. It is the last command that he gave his disciples before ascending back into heaven. It is important. It is thus called "The Great Commission."

 

By "precise formula" I meant all of the more specific definitions and terminology in the later creeds; not simply an order of names. Those scripture references are not really "formulas", but rather statements. And a doxology I would think would carry the same weight as the Commission, if an order of names was the point.

 

Your point is rather moot. Our pastor often ends with a "benediction", basically the same thing. There is a variety of verses that he uses. Here is a common one:

Jude 1:24-25 Now unto him that is able to keep you from falling, and to present you faultless before the presence of his glory with exceeding joy, To the only wise God our Saviour, be glory and majesty, dominion and power, both now and ever. Amen.

There is no precise reference to the trinity here. It is to be a reference of praise and glory to God that ends the service. What is the traditional doxology:

Praise God from whom all blessings flow.
Praise Him all creatures here below.
Praise Him above ye heavenly hosts.
Praise Father, Son, and Holy Ghost.

But Scripture can be used to end the service. Nothing is cut in stone.