1) Despite the claim that Scripture was the sole sufficient rule of faith, Protestant reformers always appealed to some type of interior illumination by the Spirit as necessary for the correct understanding of the Word of God (and also what comprised the Word of God). To put this in scholastic language, they showed that the written texts themselves were only materially sufficient as a source of truth. There was still a need for a formal principle of sufficiency (such as the inner guidance of the Holy Spirit) to make the truths of the scriptures discernable. The question of how one could know who had the guidance of the Holy Spirit was never answered to any degree of certitude and satisfaction.
2) The Protestant reformers soon discovered that the Scriptures were open to diverse interpretations. This led to tensions and divisions among the various groups of reformers. Such experience points to the difficulty with the claim that the true meaning of Scripture is clear to all “true believers,” and it shows that sola scriptura was, and has been, a principle of disunity rather than unity among Christians. The existence of [thousands of] distinct Christian denominations in the world today is a direct result of the sola scriptura doctrine.
3) Although Luther and the other Protestants saw themselves as “captive” to the Word of God, a close study shows that they often viewed the scriptural texts through the lens of their own theological doctrines and opinions. Luther was probably the most dramatic example in this regard since he was willing to use his own interpretive framework of justification by faith alone as a rationale for disparaging and even eliminating certain biblical texts as “unevangelical.” Ultimately, the Bible could be cited as an authority the only to the extent that it conformed to certain theological perspectives.
4) Despite the claim that the Bible was the sole, sufficient rule for matters of the faith, the Protestants (whether consciously or not) were always appealing to some other rule of authority. Luther came to rely on his own prophetic inspiration and, therefore, his judgments and interpretations became an authoritative rule (at least to himself and his close followers). The Calvinists and the Anglicans retained various forms of church structure and authority which provided them with something analogous to the hierarchical structure of Catholicism (even thought they claimed the Bible as the ultimate court of appeal). The Anabaptists, the Spiritualists and the Rationalists of the Radical Reformation were more subjective or “spirit-based” in their search for authority, and this led many of them to abandon the sola scriptura doctrine altogether.
5) Whether intentionally or not, the Protestants doctrine of sola scriptura launched the West into an inevitable trajectory towards subjectivism and individualism. The dangers of chaos and self-delusion are always present when there is a break from an established authority on the basis of personal claims of knowledge or illumination. Only to a limited extent, can the words of the Bible serve as a “check” against unrestrained subjectivism. While adherence to the ancient creeds has helped to maintain some semblance of unity among the major Protestant denominations, the doctrine of sola scriptura has also opened the door to all types of aberrations. Groups like Jehovah Witnesses who deny the Trinity and the divinity of Christ are a logical outcome of the “Bible only" philosophy. Contemporary secular forms of individualism and relativism can likewise be understood as part of the historical fallout of subjectivism spawned by sola scriptura doctrine. (Robert Fastiggi, “What did the Protestant Reformers Teach about Sola Scriptura?,” in Not By Scripture Alone: A Catholic Critique of the Protestant Doctrine of Sola Scriptura, ed. Robert A. Sungenis [2d ed.; State Line, Pa.: Catholic Apologetics International Publishing, Inc., 2013], 332-33, comment in square brackets added for clarification)
Further Reading:
Not By Scripture Alone: A Latter-day Saint Refutation of Sola Scriptura