Perspicuity---the notion that the Bible is sufficiently unambiguous on the whole for well-intentioned persons with Christian faith to understand with relative adequacy---is no small thing. Recognizing that none of us, even the most hermeneutically savvy, is immune from the cataract of confusion that inhibits a perfect reading of the text, evangelical Protestants nevertheless stand on the conviction that Scripture's clarity is rationally and empirically defendable. In the recently released book Journeys of Faith: Evangelicalism, Eastern Orthodoxy, Catholicism, and Anglicanism, I respond to Catholic historian Brad Gregory of Notre Dame by arguing that the doctrine of perspicuity is indeed supported by reason and by history.
The first hurdle that perspicuity must clear is the accusation that it, along with sola scriptura in general, is "perfectly circular." Logically speaking, an explanation of one's ultimate beliefs always involves a degree of circularity. The Protestant relies on Scripture to define his belief in Scripture as the supreme source of authority; the Catholic relies upon the church institution to define authority by that institution; the Muslim relies on the Quran for his Quranic religion; and the secular humanist clings to his own reason. This is not to espouse fideism---that faith is independent of reason. The foundation of our beliefs should be regularly scrutinized. Intellectual honesty requires it, as does the deepening of our faith. Nevertheless, the circular pattern of ultimate truth claims is an unavoidable function of the fact that every view stands upon some sort of epistemic ground. In other words, there is no such thing as a view from nowhere.
However, our commitment to this ground shouldn't inhibit further investigation. As imperfect interpreters, we must carefully assess and measure evidence to determine which rendering of the data is most tenable. Short of core tenets of faith, such as what we have in the Apostles' and Nicene Creeds---primary doctrines that God's people have agreed upon from time immemorial---honesty and humility requires that we remain open to doctrinal reform.
God Has Spoken
Evidence for perspicuity is based on the premise that God has spoken in order to be understood. Accordingly, the Hebrew prophets addressed themselves to regular people, not only to religious leaders (Ps. 119:105). Scripture is said to be clear enough for the "simple" to understand and benefit from its message. "The statutes of the LORD are trustworthy, making wise the simple" (Ps. 19:7). Again, we read, "The unfolding of your words gives light; it gives understanding to the simple" (Ps. 119:130).
The New Testament attests to the same emphasis by Jesus and the apostles. When Jesus encountered individuals who misunderstood doctrine, their confusion was never blamed on the imperspicuous nature of Scripture; instead, our Lord responds with statements such as "Have you not read . . ." (Matt. 12:3, 5; 21:42; 22:31), "You are wrong because you know neither the Scriptures nor the power of God" (Matt 22:29; cf. 9:13; 12:7; 15:3; 21:13; John 3:10; et al.), or even "How foolish you are, and how slow of heart to believe all that the prophets have spoken" (Luke 24:25). In each of these instances, Jesus expected his interlocutors to understand the proper meaning of Scripture.
The epistles of the New Testament offer further proof of the Bible's clarity. Paul's letters were written not to church leaders but to entire congregations, "to all the saints in Christ Jesus who are at Philippi" (Phil. 1:1; cf. 1 Cor. 1:2; Gal. 1:2). Peter addressed believers scattered throughout Asia Minor (1 Pet. 1:1). Although Peter acknowledged that Paul's letters contain some things that are hard to understand, he nowhere lets readers off the interpretive hook, but instead places the blame on the "ignorance and unstable people" who distort these passages "as they do the other Scriptures, to their own destruction" (2 Pet. 3:16).
Augustine deals with the issue of interpreting obscure passages in his work, On Christian Doctrine, describing how they reveal our sin-darkened intelligence. What's interesting is that Augustine says nothing of a church magisterium as the solution to these conundrums; rather, he portrays hard sayings as purposefully arranged by God to subdue our pride and feed charity.  Augustine's view on this point is also the biblical portrait. Scripture lacks any example of an infallible teaching office; nor does it have a revealed interest in one. Instead, it portrays an eschatological reality in which we now see but a poor reflection as in a mirror, awaiting the day when we will see truth face to face (1 Cor. 13:12). This reality should infuse the activity of biblical interpretation with a conscious balance of grace and truth, or, in Richard Baxter's words, "unity in necessary things; liberty in doubtful things; and charity in all things." We contend for the faith (Jude 1:3) while simultaneously preserving the unity of the Spirit in the bond of peace (Eph. 4:3).
What about Professor Gregory's challenge to evangelicals to furnish evidence of perspicuity in history? Here is one example: the Third Lausanne Congress on World Evangelization, held October 17 to 25, 2010, in Cape Town, South Africa. Out of 4,000 delegates, 197 nations were represented. A spectrum of churches and ethnicities were included speaking eight official languages: Arabic, Chinese, English, French, German, Portuguese, Russian, and Spanish. Centered upon a common commitment to the gospel, this global church council (including cardinals from the Catholic Church observing on behalf of the Vatican) gathered as brothers and sisters in Christ to prayerfully seek the Lord's wisdom to address the challenges and opportunities for the spread of the gospel in today's world. Christian humility born from the recognition of our sin-stained hearts and minds prevented any one church from claiming dominance over the other; nevertheless, participants engaged in discussion with an absolute regard for truth. Along with the enabling grace of the Holy Spirit, this was made possible because of the perspicuity of Scripture.
 Augustine, De Doctrina Christiana, 2.6.7. Augustine explores challenges of interpreting obscure passages in several chapters of Book II and into Book III.