One objection that is occasionally addressed to the Reformed doctrine of the perspicuity of Scripture, and also sometimes to the doctrine of the sufficiency, formal and material, of Scripture, is a request that we provide an exhaustive list of the doctrines that are necessary to salvation.
After all, we claim that all the things that are necessary to be known for salvation are clearly taught in Scripture. Some folks think this is an incredible claim unless we can provide a list of the necessary things. There are several rebuttals to this objection.
1) No one thinks the list itself is necessary to salvation
In other words, while one may need to know the essential doctrines, there is no requirement that one be able to distinguish the essential doctrines from the unessential doctrines. So, even if the Bible does somehow tell us which doctrines are essential, that list is not something that falls within the realm of the doctrine of perspicuity. Furthermore, if we reach the conclusion that the Scriptures do not provide such a list, the doctrine of the sufficiency of Scripture would simply confirm that such a list is not necessary to the rule of faith.
2) We derive the doctrine of perspicuity from Scripture deductively
In other words, we reason from the fact that the Scriptures are able to make the simple wise and specifically in order to make a person wise unto salvation, that they consequently teach with sufficient clarity all that must be known for salvation.
3) An analogy to Medicine
The field of medicine provides an analogy. Suppose you have a drug that you realize cures some particular ailment, such as malaria. There is no reason you need to know which ingredients in the medicine cure you, you just need to eat the medicine. By analogy, you do not need to know which doctrines of Scripture are essential. You just need to believe what the Scripture teaches. If you do so, that should lead you to repent of your sins and trust in Christ alone for salvation.
Christians are required to believe all of the Bible. The canonical Scriptures are our rule of faith, and only the canonical Scriptures are our rule of faith. Adherence to the rule of faith is not the way that people are saved, but it is a Christian duty. It is through believing the gospel that the Bible proclaims that sinners are justified. Our inability to identify those points that are essential should simply prod us to study Scripture more intensively and seek by prayer and study to understand it more fully.