The following is from the web page Luther, Exposing the Myth, under the heading "Sacred Scripture":
It is worth noting that while Luther claimed for himself the right to interpret scripture according to his own view, and claimed that he was intelligent enough to judge anyone and everything by scripture alone yet he openly affirms that "We cannot claim to fathom completely the meaning of a single verse of Scripture; we succeed in apprehending only the A B C of it, and even that imperfectly." - Luther, Table-talk, trans. Gustave Brunet, Paris, Garnier, 1844, pg. 288.
Luther Exposing the Myth says their stated purpose is to show that "from Luther’s own words we shall see him for what he really was, that is a rebellious apostate, who abandoned the faith and led many into apostasy from God under the guise of “reformation” in order to follow his perverse inclinations." With this quote, they attempt to show Luther contradicted himself saying he could interpret the Bible, but elsewhere claimed he could not.
Luther, Exposing the Myth cites "Luther, Table-talk, trans. Gustave Brunet, Paris, Garnier, 1844, pg. 288." First, the Table Talk isn't something Luther actually wrote. It's something he's purported to have said. Second, based on previous documentation, it's improbable that Gustave Brunet's book was actually used by Luther, Exposing the Myth (they typically have compiled their assortment of Luther quotes from various secondary English sources). Third, in regard to Gustave Brunet's French translation of the Table Talk: Les Propos de Table, Preserved Smith has provided a brief overview of this translation. According to LW 54, Brunet's French translation follows John Aurifaber's translation (LW 54, introduction). Page 288 can be found here.
In Brunet's French translation, the quote in question states,
Le docteur Juste Jonas, étant une fois à table avec le docteur Martin Luther, dit qu'il y avait dans l'Ecriture sainte une sagesse si profonde que personne ne pouvait l'étudier à fond, ni la comprendre entièrement; et le docteur Luther dit alors : « Nous sommes en effet des écoliers, car nous ne saurions prétendre à approfondir complètement un seul verset de l'Ecriture, et nous ne réussissons qu'à en saisir l'A, B, C, et même imparfaitement.The German text can be found here:
English translations are available in older versions of the Table Talk. For instance:
Dr. Jonas Justus remarked at Luther's table: There is in the Holy Scripture a wisdom so profound, that no man may thoroughly study it or comprehend it. "Ay," said Luther, "we must ever remain scholars here; we cannot sound the depth of one single verse in Scripture; we get hold but of the ABC, and that imperfectly. Who can so exalt himself as to comprehend this one line of St. Peter: 'Rejoice, inasmuch as ye are partakers of Christ's sufferings.' Here St. Peter would have us rejoice in our deepest misery and trouble, like as a child kisses the rod."In this English version, the line that begins, "Who can so exalt himself as to comprehend this one line of St. Peter..." is based on a translation of a few sentences from the next paragraph in the German version (and it can also be found in Brunet's French translation).
In context, the remark simply points out the profound depth of the Scriptures. It's not that the Scriptures can't be understood according to Luther, it's that they have a rich unfathomable depth due to the fact they are the very words of God. Note the example given to Peter's words.
This quote doesn't appear to be in LW 54, but something very similar is presented:
No. 5562: To Believe and to Comprehend Are Not the Same Spring, 1543
When Dr. Jonas said that the mind of man cannot comprehend articles of faith and that it is enough that we begin only to assent, the doctor [Martin Luther] said, “Yes, dear Dr. Jonas, if one could believe them the way they’re written, our hearts would leap for joy. That’s certain. Accordingly we won’t arrive at the place where we comprehend them. In Torgau a wretched little woman once came to me and said, ‘Ah, dear Doctor, I have the idea that I’m lost and can’t be saved because I can’t believe.’ Then I replied, ‘Do you believe, dear lady, that what you pray in the Creed is true?’ She answered with clasped hands, ‘Oh yes, I believe it; it’s most certainly true!’ I replied, ‘Then go in God’s name, dear lady. You believe more and better than I do.’
“It’s the devil who puts such ideas into people’s heads and says, ‘Ah, you must believe better. You must believe more. Your faith is not very strong and is insufficient.’ In this way he drives them to despair. We are so constructed by nature that we desire to have a conscious faith. We’d like to grasp it with our hands and shove it into our bosom, but this doesn’t happen in this life. We can’t comprehend it, but we ought to apprehend it. We should hold to the Word and let ourselves drag along in this way.” [LW 54:453].