That's a good question.
To drastically oversimplify it ("the medium affects the message"):
The whole church embraced the Apostle's Creed, so start there. For what is meant by it, see the Nicene Creed, Athanasian "creed", and the definition of Chalcedon. This means orthodox (with a small 'o') Trinitarianism.
From there, testing everything according to Scripture, decide who continued the Apostle's teaching, and *on what grounds*, whether Rome or the Reformers.
Then, decide your position on the Synod of Dordt.
There hasn't been much new by way of "heresy" since then--just periodic revivals of questions long since answered.
Shortcut: embrace the Westminster Standards, the Three Forms of Unity, or (if you reject household baptism) the 1689 LBCF. And you'll generally be in a good spot in terms of where the Apostle's teaching persists today.