Merely-Useful
Natural revelation suffices for salvation; Scripture is helpful but not strictly necessary — an Arminian-rationalist tendency rejected by the Confession.
This is a contested or rejected alternative.
The Westminster baseline on Necessity is Necessary-After-The-Fall. Personas and schools listed below hold this alternative position instead — either because they argued for it at the Assembly (like the Erastians on polity) or because they represent a receiving tradition that departed from the Standards on this point.
Other positions on Necessity
Necessary-After-The-Fall WCF
Although the light of nature and the works of creation manifest God's goodness, wisdom, and power, they are not sufficient to give the knowledge of God and his will necessary for salvation; therefore the Lord committed his revelation wholly unto writing (WCF I.1).
Unnecessary
The inner light alone suffices; Scripture is an aid, not a necessity — the position of the most radical spiritualists.