The perspicuity of Scripture is basically the doctrine which states that Scripture is plain to all who will read it. Scripture is thus not an esoteric text whereby only a select elite few could decipher its 'deeper meaning', but to all who would read or hear the text for themselves in a language they understand, they could learn from Scripture and know God from it.
Now, what does this doctrine actually mean? Does this doctrine negate the necessity of studying the Bible, especially in the original languages? To answer this and other questions, I would like to further expound on what this doctrine actually means and what it does NOT mean, and then showing the proofs for this doctrine from Scripture.
First of all, this doctrine says that the Scripture is plain to all people who truly are interested in finding out what it actually says. It does not mean that it is plain to someone who comes to Scripture with preconceived bias and conclusions, and definitely not with a superior prideful attitude towards Scripture. Secondly, this doctrine says that a person is able to know what the Scriptures say if they could understand the words and concepts as expressed in a language they can understand. It thus does not mean that the person who reads it can understand everything in the Scriptures if they do not comprehend the words used in it. This doctrine also DOES not say that all things in Scripture can be similarly understood easily, but it confesses with the Westminster Confession where it states that 'All things in Scripture are not alike plain in themselves, nor alike clear unto all; yet those things which are necessary to be known, believed, and observed, for salvation, are so clearly propounded and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them'.
So does the Scriptures support this doctrine of perspicuity? It does. First of all, we can take note that the Scriptures are written to be read, memorized (Ps. 119:11), meditated upon (Josh. 1:8, Ps. 119:97), taught (2 Tim. 3:16), applied (2 Tim. 3:16-17) and obeyed (Ps. 119:129). This is just but a sample of the verses that can be used to support the ways in which Scripture is to be used. Now, if Scriptures are to be used by Christians in such a manner, surely this presupposes that the Scriptures can be understood by those who read it, otherwise how can they be utilized in such a manner? However, could it be that perhaps the Scripture were to be read and interpreted by the clergy and then the laity would then do all these godly things, while leaving the interpretation of Scripture exclusively to the clergy?
We can see from the Scriptures how the Scripture were read and interpreted, which would inform us on whether there exists such a clergy/ laity distinction with regards to the interpretation of Scripture. First of all, during the time of Moses, the Jews were commanded to obey all the commands and the decrees of the LORD (Deut. 11:13; 11:1), to memorize them (Deut. 11:18), and to teach their children the Law of God (Deut. 11:19). Note that this command was given to all the Jews. More specifically, the command to teach their children was given to the father who is the head in the household. This shows that all the Jews are to know the Law (the given Scriptures of their time). Furthermore, since for the fathers it involves teaching the children, they must first understand the Scriptures before they teach their children. Therefore, such reading and understanding of the Scriptures is not limited to the 'clergy' of their day only, the Levites and the Aaronical priesthood, but to all Jews. Since Israel is the Old Testament Church, this represents the fact that all Christians are to know the Scripture for themselves, and also that fathers must teach their children the Scriptures.
During the time of Jesus, it was the custom to read from the Scriptures in the synagogue, as Jesus did (Lk. 4:16-17). From this, we can see that the Scripture were read to the ordinary folks, even to ordinary Jewish villagers in the town of Nazareth. Therefore, we can see that even ordinary unlearned villagers were expected to be able to understand the Scripture by themselves, in order to abide by them.
When we go to the Apostolic age, we find that the doctrine of the perspicuity of Scripture is even more explicit. Of special note is the case of the Bereans, who test the Apostle Paul's teaching by the Scriptures they had (Acts. 17: 11) and were commended for doing so by Luke who called them noble-minded. This presupposes the doctrine of perspicuity as they can only test Paul's teaching by Scripture ONLY if they can understand what Scripture teaches in the first place. Another place in the New Testament where the doctrine of perspicuity is implied in the various places where we are told to contend earnestly for the faith (Jude 1:3), which would be meaningless unless one can know the faith through reading the Scriptures. Even more explicit is the words in Scripture by Luke stating that the reason why he is writing the Gospel named after him (Lk. 1:4) is so that 'you may have certainty concerning the things you [Theophilus] have been taught'. Also, in 1 Jn. 5:13, a verse used often to talk about having assurance of salvation, the whole book of 1 John was written so that believers can know that they have eternal life. Therefore, the Scripture are written so that we would know for sure the things which God wants us to know, which is thus plain to us.
Now, another aspect of the doctrine of perspicuity is that, although it is plain to all, those who are wise in their eyes and who thus approach the Scripture with bias unbelief, together with those whose eyes are blinded, would not be able to discern the true meaning of Scripture. The former is found in 1 Cor. 1:18-25, especially in verse 20 where it is stated that God has made foolish the wisdom of the wise, which expresses itself through them ridicuing the way of salvation through the Cross of our Lord Jesus Christ. Since through the Scriptures alone, Man can know God and be saved (Rom. 10:14-17), therefore those who are wise in their eyes would not be able to understand Scripture. The latter is seen in 2 Cor. 4:3-4 where the god of this world or Satan has deceived and veiled the eyes of the unbelievers who do not know God, so that they can't see Him nor hear the Gospel which is found in Scripture.
Then, how can Man truly know and understand the Scriptures? Although the Scriptures do tell us that each of us can know the Scriptures through the commands of Scripture and the examples in Scripture as seen above, we have also seen that the Scriptures are veiled towards those who are wise in their own eyes, which include Christians who place their traidtion above Scripture and all non-Christians. Of course, it can be said that Christians who plce their traditions above Scripture should humble themselves and submit to the Word of God and become babes (Lk. 10:21) so that they might know the Scriptures. However, what about the non-Christians as stated in 2 Cor. 4:3-4? Where does that leave the doctrine of perspicuity?
The answer is to be found in 1 Cor. 2:12-16. In this passage, we can see that the key to understanding Scripture is through the infilling of the Holy Spirit who will teach us Christians all things and guide us into all truth (Jn. 14:26; 1 Cor. 2:13). Therefore, the doctrine of perspicuity of Scripture is indeed correct, with the clarification that the Scriptures though perspicuous but can only be rightly understood by those who are the children of God, and thus have the Holy Spirit's guidance in understanding Scripture.
Now, this is not to say that unbelievers cannot intellectually know the meaning of Scripture. In fact, it is precisely because they can intellectual understand the claims of Scripture that they scoff and reject the Gospel, as it is foolishness to them (1 Cor. 1:24). Therefore, what is meant by the fact that they cannot know the Scriptures is that they could not comprehend the reasonableness of the Christian faith. In fact, since God is the only Truth, and Christ is the Logos of the World (Jn. 1:1), this shows that all non-Christians are actually intellectually deficient when it comes to things of eternity (though they could be genuises in other fields), which spring from their spiritual rebellion against God.
Last but not least, the doctrine of perspicuity says that not all things in the Scripture are 'alike plain in themselves', which means that some things in Scripture are easier to understand than others. The classic proof text for this can be found in 2 Peter 3:16, where some of the Apostle Paul's teaching is harder to understand than others. Therefore, this clarification on the doctrine of perspicuity is proved.
In the next installment, we would look at some objections to the doctrine of the perspicuity of Scripture.
 Westminster Confession of Faith, Chapter I Sentence VII. As seen at http://www.reformed.org/documents/westminster_conf_of_faith.html