“The Reformation principle of sola Scriptura has to do with the sufficiency of Scripture as our supreme authority in all spiritual matters. Sola Scriptura simply means that all truth necessary for our salvation and spiritual life is taught either explicitly or implicitly in Scripture. It is not a claim that all truth of every kind is found in Scripture… [it is] the perfect and only standard of spiritual truth, revealing infallibly all that we must believe in order to be saved and all that we must do in order to glorify God. That–no more, no less–is what sola Scriptura means.” (John MacArthur)
For more on this subject, please visit the Systematic Theology page.