Perspicuity of Scripture is the belief that everything pertaining to salvation is understandable to the common man from a simple reading of Scripture when God opens a person’s eye to the truth.
Notice though that God must open a person’s eyes. Until the Lord opens their eyes nothing they read may make sense. That is why things that seem so plain to a believer is confusing to a non-believer.
1 Corinthians 2:14 The natural person does not accept the things of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned. ESV
2 Corinthians 3:14-16
14 But their minds were hardened. For to this day, when they read the old covenant, that same veil remains unlifted, because only through Christ is it taken away.
15 Yes, to this day whenever Moses is read a veil lies over their hearts.
16 But when one turns to the Lord, the veil is removed. ESV
43 Why do you not understand what I say? It is because you cannot bear to hear my word.
44 You are of your father the devil, and your will is to do your father’s desires. He was a murderer from the beginning, and has nothing to do with the truth, because there is no truth in him. When he lies, he speaks out of his own character, for he is a liar and the father of lies.
45 But because I tell the truth, you do not believe me.
46 Which one of you convicts me of sin? If I tell the truth, why do you not believe me?
47 Whoever is of God hears the words of God. The reason why you do not hear them is that you are not of God.” ESV
The Roman Catholic Church claimed that Scripture was not clear to the common man. They felt that only the church leaders could interpret Scripture. That was why they didn’t want man to have a copy of the Scriptures in their own language. The reformers fought long and hard (often to their death) to let man have and read Scriptures on their own.
Westminster Confession of Faith (1.7)
All things in Scripture are not alike plain in themselves, nor alike clear unto all (2 Pet. 3:16); 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 (Ps. 119:105, 130).
Does this mean that all of Scripture is equally clear or that there aren’t hard portions?
No, it doesn’t. Scripture is such that anyone could spend a lifetime studying it and still not get all there is to know from the Scriptures.
Even Peter stated that Paul was hard to understand.
2 Peter 3:15-16
15 And count the patience of our Lord as salvation, just as our beloved brother Paul also wrote to you according to the wisdom given him,
16 as he does in all his letters when he speaks in them of these matters. There are some things in them that are hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other Scriptures. ESV