I have been considering this a lot recently. What is the minimum requirement for salvation? I see a lot of gatekeeping on all sides of the discussion so I've been trying to get to the bottom of what the Bible says is necessary for salvation. Looking at examples of people in both the Old and New Testaments. Take Rahab for example, she basically only knew that God had been destroying other nations and she and her family were saved. In the New Testament we have examples like the Ethiopian eunuch who it says was taught starting in Isaiah about who the Christ is and what He did. And I keep coming back to 1 Corinthians 15:1-11 where Paul describes the Gospel as he proclaimed it to the Corinthians.

1 Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand, 2 and by which you are being saved, if you hold fast to the word I preached to you—unless you believed in vain. 
3 For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, 4 that he was buried, that he was raised on the third day in accordance with the Scriptures, 5 and that he appeared to Cephas, then to the twelve. 6 Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. 7 Then he appeared to James, then to all the apostles. 8 Last of all, as to one untimely born, he appeared also to me. 9 For I am the least of the apostles, unworthy to be called an apostle, because I persecuted the church of God. 10 But by the grace of God I am what I am, and his grace toward me was not in vain. On the contrary, I worked harder than any of them, though it was not I, but the grace of God that is with me. 11 Whether then it was I or they, so we preach and so you believed. 

The Holy Bible: English Standard Version, (Wheaton, IL: Crossway Bibles, 2016), 1 Co 15:1–11.

I think the conclusion that I have tentatively come to is that the foundation of the Gospel of Jesus the Christ is that we are dead in our sin with no hope, Jesus came and lived a perfect life, He was killed as a payment for our sin, and He rose again defeating death and giving us hope for the resurrection. Now there are other things besides these that I see as important, however I am currently reluctant to call someone an unbeliever so long as they hold to these foundational doctrines.