All of the hoopla surrounding Stellman has reminded me of the importance of this question: how do we build our theology? And if we are tempted by the Tiber, how do we go about resolving the questions that arise?
I think that the Bayly brothers have some excellent advice. I would modify it a bit by saying that anyone tempted by the Tiber should concentrate on two issues: justification and Scripture.
On either of these issues, a person should scour the Reformed tradition to see if there are good answers to their specific questions. Do not neglect the older authors, either. On justification, one should read (at least!) the following three books: Owen, Buchanan, and Fesko. It wouldn’t hurt either to do some digging in the OPC study committee report to learn why union with Christ does not make imputation redundant, but rather ensures that imputation is not a legal fiction. These resources would be a bare minimum of what a person should read.
On Scripture, I cannot even stress the importance of Whitaker strongly enough. If you read no other book on Scripture, read Whitaker. This is BY FAR the best book ever written on Scripture from the Protestant perspective. And there are plenty of other great books out there. I would also recommend volume 2 of Muller’s Post-Reformation Reformed Dogmatics (which is out of print, so you’ll have to find it on Bookfinder or ABEBooks). Pay particular attention to the perspecuity of Scripture, as this is the primary sticking point between Rome and Geneva.
One final word to young theologians: build your theology from the older masters. The reason for this is two-fold: 1. Not only are their works tried and true, having been analyzed more often than modern works, but also, 2. The fountain of the Reformed faith is less likely to be quirky than later theologians. So build your theology on Calvin, Turretin, a’Brakel, and Bavinck. This is not to say that modern theologians like Horton and Kelly should not be read. They should be. However, those guys would be the first to admit that you should read the older theologians first. This will give you a stronger and more centered root to your theology.