Evangelical theological method is so concerned to root revelation in the Bible because most of them in theory, and virtually all of them in practice, have lost any sense of the Holy Spirit doing anything beyond illuminating their reading of the text. Without any space in their theology for the idea that the Holy Spirit might say anything to them directly, evangelicals only have the Bible left as a locus for supernatural revelation. Yet since it is a text, the Bible is therefore suitable for rational investigation (thus the dominance of the historical-grammatical hermeneutic in Evangelical thought). Thus exegesis becomes the mode of divine communication today, and the role of the Holy Spirit is to guide and guarantee the correctness of that exegesis and the subsequent application.
This fundamental shift to cessationism (whether doctrinal or merely practical) sets Evangelical thought into a very different situation from that of the patristic church. Both Evangelicalism and the early church saw the Bible (from which the early creeds and formulae were derived) as providing the rule of faith. However, the early church used this rule to guard against misguided readings that claimed to come from some kind of new, supernatural revelation of otherworldly wisdom, whereas Evangelicalism uses this rule to guard against encroachments of modern secularism. The former is guarding against heresy and syncretism, the latter is guarding against unbelief. The early church did not use the Rule of Faith (i.e. the creedal statements derived from and summarizing the Bible) to combat pagan religions, since it was completely inapplicable to them. It was no use to say to a worshipper of Mithras “Mithraism violates our Rule of Faith, so it must be rejected,” because he would not listen to such a claim. First he had to encounter Jesus and be converted before the Rule of Faith would have any claim on him. Only once he was a Christian could the Rule of Faith be applied to him and his theology. Evangelicals, on the other hand, seem to want to use the Rule of Faith (i.e. the Bible and statements derived from and summarizing it) to fight against secularism. They protest that secular culture does not conform to the Bible and insist that Western civilization needs to return the Bible to its proper place so that the truth of the Bible can be heard and thus people can encounter Jesus. This is, of course, madness and doomed to failure, since the modern non-Christian has no more reason to accept the Bible’s authority than the ancient Mithraite.
The essential difference here is that the early church and the heretics it fought had in common a supernatural faith that included the ongoing speech of the Spirit, whereas the evangelical church has boiled all supernatural revelation down into Scripture and its interpretation and relies on this over against the naturalist assumptions of its opponents. The early church’s use of the regulating function of Scripture and creed was aimed to judge purported Spirit revelations to determine their true source. The Evangelical church’s use of this regulating function is conflated with the revelatory function (which itself has been constricted only to Scripture) and is used to resist and counter beliefs that do not claim the same source at all.