Once have I spoken - That is, in vindicating myself. He had once spoken of God in an irreverent and improper manner, and he now saw it.
But I will not answer - I will not now answer, as I had expressed the wish to do. Job now saw that he had spoken in an improper manner, and he says that he would not repeat what he had said.
Yea, twice - He had not only offended once, as if in a thoughtless and hasty manner, but he had repeated it, showing deliberation, and thus aggravating his guilt. When a man is brought to a willingness to confess that he has done wrong once, he will be very likely to see that he has been guilty of more than one offence. One sin will draw on the remembrance of another; and the gate once open, a flood of sins will rush to the recollection. It is not common that a man can so isolate a sin as to repent of that alone, or so look at one offence against God as not to feel that he has been often guilty of the same crimes.
But I will proceed no further - Job felt doubtless that if he should allow himself to speak again, or to attempt now to vindicate himself, he would be in danger of committing the same error again. He now saw that God was right; that he had himself repeatedly indulged in an improper spirit, and that all that became him was a penitent confession in the fewest words possible. We may learn here:
(1) That a view of God is fitted to produce in us a deep sense of our own sins. No one can feel himself to be in the presence of God, or regard the Almighty as speaking to him, without saying, "Lo I am vile? There is nothing so much fitted to produce a sense of sinfulness and nothingness as a view of God.
(2) The world will be mute at the day of judgment. They who have been most loud and bold in vindicating themselves will then be silent, and will confess that they are vile, and the whole world "will become guilty before God." If the presence and the voice of God produced such an effect on so good a man as Job, what will it not do on a wicked world?
(3) A true penitent is disposed to use but few words; "God be merciful to me a sinner," or, "lo, I am vile," is about all the language which the penitent employs. He does not go into long arguments, into metaphysical distinctions, into apologies and vindications, but uses the simplest language of confession, and then leaves the soul, and the cause, in the hands of God.
(4) Repentance consists in stopping where we are, and in resolving to add no more sin. "I have erred," is its language. "I will not add to it, I will do so no more," is the immediate response of the soul. A readiness to go into a vindication, or to expose oneself to the danger of sinning again in the same way, is an evidence that there is no true repentance. Job, a true penitent, would not allow himself even to speak again on the subject, lest he should be guilty of the sin which he had already committed.
(5) In repentance we must be willing to retract our errors, and confess that we were wrong - no matter what favorite opinions we have had, or how tenaciously and zealously we have defended and held them. Job had constructed many beautiful and eloquent arguments in defense of his opinions; he had brought to bear on the subject all the results of his observation, all his attainments in science, all the adages and maxims that he had derived from the ancients, and from a long contact with mankind, but he was now brought to a willingness to confess that his arguments were not solid, and that the opinions which he had cherished were erroneous. It is often more difficult to abandon opinions than vices; and the proud philosopher when he exercises repentance has a more difficult task than the victim of low and debasing sensuality. His opinions are his idols. They embody the results of his reading, his reflections, his conversation, his observation, and they become a part of himself. Hence, it is, that so many abandoned sinners are converted, and so few philosophers; that religion spreads often with so much success among the obscure and the openly wicked, while so few of the "wise men of the world" are called and saved.

Once have I spoken - See on Job 42:3 (note), etc.
I will proceed no farther - I shall attempt to justify myself no longer; I have spoken repeatedly; and am confounded at my want of respect for my Maker, and at the high thoughts which I have entertained of my own righteousness. All is impurity in the presence of thy Majesty.

Once have I spoken; but I will not answer,.... Some think this refers to what he had just now said of his vileness, he had owned that, and that was all he had to say, or would say, he would give no other answer; Jarchi says, some suppose he has respect to his words in Job 9:22;
yea, twice; but I will proceed no further; the meaning seems to be, that he who had once and again, or very often, at least in some instances, spoken very imprudently and indecently, for the future would take care not to speak in such a manner: for this confession was not quite free and full; and therefore the Lord takes him in hand again, to bring him to make a more full and ingenuous one, as he does in Job 42:1.

Once . . . twice--oftentimes, more than once (Job 33:14, compare with Job 33:29; Psalm 62:11):
I have spoken--namely, against God.
not answer--not plead against Thee.

Answer - Speak again; I will contend no more with thee. Twice - Often, the definite number being used indefinitely.