The following are quotes from the church fathers affirming the clarity of the Scriptures. All quotes are taken from http://www.ccel.org/fathers2/ (emphasis mine):
Justin
Martyr
And I replied, "I would not bring forward these proofs, Trypho, by which I am aware those who worship these [idols] and such like are condemned, but such [proofs] as no one could find any objection to. They will appear strange to you, although you read them every day; so that even from this fact we understand that, because of your wickedness, God has withheld from you the ability to discern the wisdom of His Scriptures; yet [there are] some exceptions, to whom, according to the grace of His long-suffering, as Isaiah said, He has left a seed of salvation, lest your race be utterly destroyed, like Sodom and Gomorrah. Pay attention, therefore, to what I shall record out of the holy Scriptures, which do not need to be expounded, but only listened to.
-Justin Martyr, Dialogue of Justin, ch.55
http://www.ccel.org/fathers2/ANF-01/anf01-48.htm#P4489_937912
And that the prophet whose name was changed, Jesus [Joshua], was strong and great, is manifest to all. If, then, we know that God revealed Himself in so many forms to Abraham, and to Jacob, and to Moses, how are we at a loss, and do not believe that, according to the will of the Father of all things, it was possible for Him to be born man of the Virgin, especially after we have such Scriptures, from which it can be plainly perceived that He became so according to the will of the Father?
-Justin Martyr, Dialogue of Justin, ch.75
http://www.ccel.org/fathers2/ANF-01/anf01-48.htm#P4723_1011892
Irenaeus
A sound mind, and one which does not expose its possessor to danger, and is devoted to piety and the love of truth, will eagerly meditate upon those things which God has placed within the power of mankind, and has subjected to our knowledge, and will make advancement in [acquaintance with] them, rendering the knowledge of them easy to him by means of daily study. These things are such as fall [plainly] under our observation, and are clearly and unambiguously in express terms set forth in the Sacred Scriptures.
-Irenaeus, Against Heresies 2.27.1
http://www.ccel.org/fathers2/ANF-01/anf01-59.htm#P7131_1850249
But that I may not be thought to avoid that series of proofs which may be derived from the Scriptures of the Lord (since, indeed, these Scriptures do much more evidently and clearly proclaim this very point), I shall, for the benefit of those at least who do not bring a depraved mind to bear upon them, devote a special book to the Scriptures referred to, which shall fairly follow them out [and explain them], and I shall plainly set forth from these divine Scriptures proofs to [satisfy] all the lovers of truth.
-Irenaeus, Against Heresies 2.35.4
http://www.ccel.org/fathers2/ANF-01/anf01-59.htm#P7278_1930324
"For, touching the resurrection of the dead," He says, "have ye not read that which was spoken by God, saying, I am the God of Abraham, the God of Isaac, and the God of Jacob? And He added, "He is not the God of the dead, but of the living; for all live to Him." By these arguments He unquestionably made it clear, that He who spake to Moses out of the bush, and declared Himself to be the God of the fathers, He is the God of the living.
-Irenaeus, Against Heresies 4.5.2
http://www.ccel.org/fathers2/ANF-01/anf01-62.htm#P8055_2221272
Theophilus
of
But do you also, if you please, give reverential attention to the prophetic Scriptures, and they will make your way
plainer for escaping the eternal punishments, and obtaining the eternal prizes of God. For He who gave the mouth for speech, and formed
the ear to hear, and made the eye to see, will examine all things, and will judge righteous judgment, rendering merited awards to
each.
-Theophilus of
http://www.ccel.org/fathers2/ANF-02/anf02-41.htm#P1399_405324
Clement
of
But if from any creature they received in any way whatever the seeds of the Truth, they did not nourish them; but committing them to a barren and reinless soil, they choked them with weeds, as the Pharisees revolted from the Law, by introducing human teachings,-the cause of these being not the Teacher, but those who choose to disobey. But those of them who believed the Lord's advent and the plain teaching of the Scriptures, attain to the knowledge of the law; as also those addicted to philosophy, by the teaching of the Lord, are introduced into the knowledge of the true philosophy: "For the oracles of the Lord are pure oracles, melted in the fire, tried in the earth, purified seven times."
–Clement of
http://www.ccel.org/fathers2/ANF-02/anf02-67.htm#P8703_2430399
And this signified that the Scripture is clear to all, when taken according to the bare reading; and that this is the faith which occupies the place of the rudiments. Wherefore also the figurative expression is employed, "reading according to the letter; "while we understand that the gnostic unfolding of the Scriptures, when faith has already reached an advanced state, is likened to reading according to the syllables.
-Clement of
http://www.ccel.org/fathers2/ANF-02/anf02-67.htm#P8931_2507342
Tertullian
These are a few testimonies out of many; for we do not pretend to bring up all the passages of Scripture, because we have a tolerably large accumulation of them in the various heads of our subject, as we in our several chapters call them in as our witnesses in the fulness of their dignity and authority. Still, in these few quotations the distinction of Persons in the Trinity is clearly set forth. For there is the Spirit Himself who speaks, and the Father to whom He speaks, and the Son of whom He speaks.
-Tertullian, Against Praxeas, ch.11
http://www.ccel.org/fathers2/ANF-03/anf03-43.htm#P10518_2950060
Theonas of
Let
no day pass by without reading some portion of the Sacred Scriptures, at such convenient hour as offers, and giving some space to
meditation. And never cast off the habit of reading in the Holy Scriptures; for nothing feeds the soul and enriches the mind
so well as those sacred studies do.
- Theonas of
http://www.ccel.org/fathers2/ANF-06/anf06-72.htm#P2857_747907
Lactantius
For this is especially the cause why, with the wise and the learned, and the princes of this world, the sacred Scriptures are without credit, because the prophets spoke in common and simple language, as though they spoke to the people. And therefore they are despised by those who are willing to hear or read nothing except that which is polished and eloquent; nor is anything able to remain fixed in their minds, except that which charms their ears by a more soothing sound.
-Lactantius, The Divine Institutes 5.1
http://www.ccel.org/fathers2/ANF-07/anf07-08.htm#P1724_691511
Hence, when learned men have applied themselves to the religion of God, unless they have been instructed by some skilful teacher, they do not believe. For, being accustomed to sweet and polished speeches or poems, they despise the simple and common language of the sacred writings as mean. For they seek that t which may soothe the senses. But whatever is pleasant to the ear effects persuasion, and while it delights fixes itself deeply within the breast. Is God, therefore, the contriver both of the mind, and of the voice, and of the tongue, unable to speak eloquently? Yea, rather, with the greatest foresight, He wished those things which are divine to be without adornment, that all might understand the things which He Himself spoke to all.
-Lactantius, The Divine Institutes 6.21
http://www.ccel.org/fathers2/ANF-07/anf07-09.htm#P2373_981173
Perspicuity of the Scriptures
According to
The Church Fathers
(Part 1)