Are those things which are necessary to be known, believed, and observed for salvation clearly propounded, open 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?
Hebrews 5:11-6:1 Concerning [Jesus] we have much to say, and it is hard to explain, since you have become dull of hearing. For though by this time you ought to be teachers, you have need again for someone to teach you the elementary principles of the oracles of God, and you have come to need milk and not solid food. For everyone who partakes only of milk is not accustomed to the word of righteousness, for he is an infant. But solid food is for the mature, who because of practice have their senses trained to discern good and evil. Therefore leaving the elementary teaching about the Christ, let us press on to maturity, not laying again a foundation of repentance from dead works and of faith toward God, of instruction about washings and laying on of hands, and the resurrection of the dead and eternal judgment.
1 Corinthians 2:1-5, 3:1-2 When I came to you, brothers, I did not come with eloquence or superior wisdom as I proclaimed to you the testimony about God. For I resolved to know nothing while I was with you except Jesus Christ and him crucified. I came to you in weakness and fear, and with much trembling. My message and my preaching were not with wise and persuasive words, but with a demonstration of the Spirit's power, so that your faith might not rest on men's wisdom, but on God's power... Brothers, I could not address you as spiritual but as worldly - mere infants in Christ. I gave you milk, not solid food, for you were not yet ready for it. Indeed, you are still not ready.
Faith and repentance from dead works encompass that which are necessary to be known, believed, and observed for salvation. These are elementary teachings of God's word, given to the immature because:
1. they are not ready for mature wisdom or eloquence.
2. so that they would first learn to rest their faith in God's power.
In other words, God's word regarding the elementary teachings of Gods word - including that which is necessary to be known, believed, and observed for salvation - are of such a nature that even the immature can understand them, due to the clarity and lack of eloquence with which they are communicated (indeed, they cannot understand the more spiritual things). This is exactly the Protestant understanding of the doctrine of perspicuity.