Clearly, there is no single, cookie-cutter evangelistic statement. There isn't just one, authorized way of communicating the gospel. That's why the summary of the gospel in 1 Corinthians 15 is a bit long, the Philippian jailer is told one thing, the Ethiopian eunuch is told something else, the conversation Jesus had with the two men following His resurrection must have taken some time, and so forth. A variety of considerations naturally come into play which explain why different statements were made in each of these cases. But I don't see why this implies a problem for the Protestant. None of this implies Scripture doesn't distinguish between essential and non-essential doctrines. The statements in the above passages are consistent with one another and touch on univocal elements.
A Protestant, to be consistent with Scripture as his ultimate rule of faith, could go through Scripture and find out what was preached when the apostles witnessed and what else in Scripture is said to be related to the gospel and salvation. He could try to compile a comprehensive list. This would make for a useful exercise, but given that the gospel can be communicated by various statements, it isn't necessary. One doesn't have to read the whole New Testament to be saved. The Corinthian church didn't have to have the "second" letter from Paul to know the gospel outlined in the "first." Knowledge of a few passages suffices, though the more you know, the better.
Protestants could also just suggest that one should believe all of Scripture - if one does this, there is no problem as to what is essential and non-essential. This response in particular strikes me as a bit implausible, though, for while all Scripture is useful, there are fundamentals which the apostles encouraged new believers to drink as milk and yet chastised other believers for not being able to move beyond. Don't be unreasonable in your expectations of a new believer's ability to understand meaty doctrine.
Obviously, we should believe all of Scripture, and all of Scripture is understandable. But Scripture is a complex communication of interrelated doctrines, some of which are implicit. Memorizing Scripture is one thing, systematizing all the inferences is another. Does any professing Christian claim to have attained this? Is to too far to assert that we don't have the capacity - now, at any rate - to believe all of Scripture at once? Does this not indicate certain content should receive priority when witnessing to an unbeliever?
This is all pretty standard, but it brings up another point. I've been advocating that Scripturalists update the subject matter of their arguments, and in the vein of continuing to do so on this blog, I thought I'd apply the distinction between occurrent and dispositional beliefs here.
An occurrent belief is a belief one has, considers, entertains, etc. at a given time. A dispositional belief is a belief one would [or, to give a necessitarian spin to this (link), could consistently be imagined to] have under certain circumstances - say, if one asked a person a question about whether or not he believes some proposition.
So let's look at the discussion of essential and nonessential doctrines from a different angle. Does everything one could list that I "would" need to agree with in order to be saved actually need to be an occurrent belief rather than a dispositional one? The answer is negative. When a believer sleeps, he doesn't usually, at least in my experience, actively believe "Jesus died and was raised for my sins." He's disposed to believe that. And we don't become unbelievers when we [occurrently] think something other than "Jesus died and was raised for me." All of this also indicates that even a Scripturalist who sincerely believes that "a person is what he thinks" must take "thinks" in a dispositional sense, so he should have no problem accepting this distinction.
However, in these cases, the actual or occurrent belief that "Jesus died and was raised for my sins" had already occurred at least once prior to my sleeping or thinking about something else. A better question is: do all propositions relating to the gospel need to have been occurrent at some prior time in order for one to be currently disposed to believe all of them? I don't see why. The burden of proof would be on the one who believes this to be the case to explain why.
Of course, I'm not saying one shouldn't entertain actual thoughts about the gospel. Less trivially, we can't know who is disposed to believe what. We have to act based on what we believe to be the case. This bears on the question of whether we should preach the whole counsel of God. I sometimes hear the argument that Christians should just list a minimal amount of propositions needed to be believed for salvation. That way, the audience isn't exposed to what I guess the arguers would call unnecessary potential obstacles to belief.
But in considering the above distinction between occurrent and dispositional beliefs, as witnesses, evangelists, and apologists of God's word, we only become aware that those to whom we are speaking actually were disposed to believe some doctrine when we actually confront them with it to see if they occurrently accept it, reject, or require clarification of it.
If one rejects a non-essential doctrine, while that doesn't necessarily mean the person isn't saved, the situation bears correction and watching. Christians make mistakes, but they should be teachable. It helps when the so-called teachers aren't constantly accusatory and defensive, which seems to be the case in many apologetic discussions. But sometimes, disagreements are never settled. That's just a fact of life we have to deal with. Sanctification is a process.
To the main point. If one rejects an essential doctrine, that's how we know he wasn't disposed to believe it and how we know he can't occurrently believe the gospel. If he accepts the essential doctrine, then we would have prima facie grounds - and here, Scripturalism needs to update its epistemology to account for kinds of justified belief other than infallibilistic - for believing they already had the disposition to believe it.
This point is relevant to cases where certain parts of the gospel may have been left unsaid in an evangelistic encounter, for even as, in that case, we could not have [as strong] grounds for believing that the audience became or were believers, for we would have no evidence of their dispositions toward what was left unsaid, God could know whether He had disposed them to believe. They could be saved after all.
Again, this doesn't discount or discourage us from activity, for we don't have access to this divine knowledge, assuming it is divinely known. We work with what we have. But that it is a possibility at all is of some note in a discussion about what must one "believe" to be saved.