"He shall see of the travail of his soul, and shall be satisfied: by his knowledge shall my righteous servant justify many; for he shall bear their iniquities."
The One who shall see the travail is the Father. The Father will be "satisfied" that the suffering of the Son, the payment made for mankind's debt of errors, has been sufficient to then inform the Son about salvation. Without descending below, the Son would have been unable to comprehend what mankind needs to overcome. Therefore Christ's suffering needed to be complete.
Upon receiving the full "wrath" of sin, Christ was then able to know how to overcome all that mankind must overcome to return to the presence of the Father. It is "by His knowledge" that Christ is able to "justify many." He possesses the knowledge, has the experience and suffered "for all" so that they might be instructed by Him. (D&C 19:16-17.) He knows. He comprehends. By the things He suffered, He gained all that is needed to redeem, comfort and succor any man or woman in their extremity. (Heb. 2:18.)
This great burden was, however, merely His preparation; and not His completion. (D&C 19:19.) He now uses His "knowledge" to "succor" and tutor each soul who will permit Him to minister to them. (Alma 7:11-12.) The most complete description of what He suffered and what He gained is set out in my testimony in Come, Let Us Adore Him.
Christ has gained "knowledge" which will save each of us, no matter what we are called to pass through, if we will come to Him, heed what He tells us, and follow His encouraging counsel. There is no depth we descend to which He does not already comprehend, having been there before us. (D&C 122:7-8.)
To overcome all sin ourselves, we must accept His guidance and counsel. His comfort alone will rid us of our guilt. He knows how to shed the pains of sin, because He has first shed them, and therefore knows what must be done. Only in this way can we relieve ourselves of the suffering which is felt when an unclean person is exposed to God's presence. (Mormon 9: 4-5.) He can lead you to cleansing, because He has been made completely filthy and covered with the wrath of God. (D&C 19:15-18.)
His "preparations" are complete. He can "succor" you back to God's presence. But you must choose to allow Him to use this hard won "knowledge to justify you" before the Father. He has borne your infirmities before you bear them. He knows how to heal from them. There is nothing which you are called to pass through that He does not already comprehend. It is this great "knowledge" which renders Him the greatest, "most intelligent of them all." (Abraham 3:19.) He now has no perplexity from sin.