A simple quick read overview might be this excerpt:

“Therefore, Christian reader, thou wilt find in my earliest books and writings how many points of faith I then, with all humility, yielded and conceded to the pope, which since then I have held and condemned for the most horrible blasphemy and abomination, and which I would have to be so held and so condemned forever. Amen.”

Here is the letter in it’s original context, translated into English:

_Dr. Martin Luther to the Christian Reader_
by Dr. Martin Luther, 1545
Published in:
_Works of Martin Luther_
Adolph Spaeth, L.D. Reed, Henry Eyster Jacobs, et Al., Trans. & Eds.
(Philadelphia: A. J. Holman Company, 1915), Vol. 1, pp. 10-11.

DR. MARTIN LUTHER TO THE CHRISTIAN READER

EDITION OF 1545

Above all things I beseech the Christian reader and beg him  for
the sake of our Lord Jesus Christ, to read my earliest books  very
circumspectly and with much pity, knowing that before now I  too
was a monk, and one of the right frantic and raving papists.  When
I took up this matter against Indulgences, I was so full and
drunken, yea, so besotted in papal doctrine that, out of my great
zeal, I would have been ready to do murder — at least, I would
have been glad to see and help that murder should be done — on
all who would not be obedient and subject to the pope, even to
his smallest word.

Such a Saul was I at that time; and I meant it right earnestly;
and there are still many such to-day. In a word, I was not such a
frozen and ice-cold champion of the papacy as Eck and others of
his kind have been and still are. They defend the Roman See more
for the sake of the shameful belly, which is their god, than
because they are really attached to its cause. Indeed I am wholly
of the opinion that like latter-day Epicureans, they only laugh at
the pope. But I verily espoused this cause in deepest earnest and
in all fidelity; the more so because I shrank from the Last Day
with great anxiety and fear and terror, and yet from the depths of
my heart desired to be saved.

Therefore, Christian reader, thou wilt find in my earliest books
and writings how many points of faith I then, with all humility,
yielded and conceded to the pope, which since then I have held and
condemned for the most horrible blasphemy and abomination, and
which I would have to be so held and so condemned forever. Amen.

Thou wilt therefore ascribe this my error, or as my opponents
venomously call it, this inconsistency of mine, to the time, and
to my ignorance and inexperience. At the beginning I was quite
alone and without any helpers, and moreover, to tell the truth,
unskilled in all these things, and far too unlearned to discuss
such high and weighty matters. For it was without any intention,
purpose, or will of mine that I fell, quite unexpectedly, into
this wrangling and contention.  This I take God, the Searcher of
hearts, to witness.

I tell these things to the end that, if thou shalt read my books,
thou mayest know and remember that I am one of those who, as St.
Augustine says of himself, have grown by writing and by teaching
others, and not one of those who, starting with nothing, have in a
trice become the most exalted and most learned doctors. We find,
alas! many of these self-grown doctors; who in truth are nothing,
do nothing and accomplish nothing, are moreover untried and
inexperienced, and yet, after a single look at the Scriptures,
think themselves able wholly to exhaust its spirit.

Farewell, dear reader, in the Lord. Pray that the Word may be
further spread abroad, and may be strong against the miserable
devil. For he is mighty and wicked, and just now is raving
everywhere and raging cruelly, like one who well knows and feels
that his time is short, and that the kingdom of his Vicar, the
Antichrist in Rome, is sore beset. But may the God of all grace
and mercy strengthen and complete in us the work He has begun, to
His honor and to the comfort of His little flock. Amen.