The servant's knowledge, or the people's knowledge?
In the case of this phrase, a study of parallel versions is very elucidating. The New International Version, for example, renders the text this way and includes a footnote:
After he has suffered, he will see the light of life and be satisfied; by his knowledgec my righteous servant will justify many, and he will bear their iniquities.
c Or by knowledge of him
In other words, instead of the knowledge possessed by the the servant saving the rebels, it is knowledge of the the servant possessed by the rebels which saves them. Paul however, suggests that this can be understood both ways, saying in Galatians 4:8-9
Formerly when you did not know God, you were enslaved to beings that by nature are not gods at all. But now that you have come to know God (or rather to be known by God), how can you turn back again to the weak and worthless basic forces? Do you want to be enslaved to them all over again?
Thus we can see, that even from the non-Christian, there is at least some basis for understanding this passage more than one (or both) ways: The people are saved through their knowledge of the messiah and/or by the messiah's knowledge of the rebels. The NET bible translates the text thus and includes the following notes:
Having suffered, he will reflect on his work, he will be satisfied when he understands what he has done. “My servant will acquit many, for he carried their sins.
Translators Note: Heb “he will acquit, a righteous one, my servant, many.” צַדִּיק (tsadiq) may refer to the servant, but more likely it is dittographic (note the preceding verb יַצְדִּיק, yatsdiq). The precise meaning of the verb (the Hiphil of צָדַק, tsadaq) is debated. Elsewhere the Hiphil is used at least six times in the sense of “make righteous” in a legal sense, i.e., “pronounce innocent, acquit” (see Exod 23:7; Deut 25:1; 1 Kgs 8:32 = 2 Chr 6:23; Prov 17:15; Isa 5:23). It can also mean “render justice” (as a royal function, see 2 Sam 15:4; Ps 82:3), “concede” (Job 27:5), “vindicate” (Isa 50:8), and “lead to righteousness” (by teaching and example, Dan 12:3). The preceding context and the next line suggest a legal sense here. Because of his willingness to carry the people’s sins, the servant is able to “acquit” them.
Study Note: Some (e.g., H. M. Orlinsky, “The So-called ‘Suffering Servant’ in Isaiah 53,22,” VTSup 14 : 3-133) object to this legal interpretation of the language, arguing that it would be unjust for the righteous to suffer for the wicked and for the wicked to be declared innocent. However, such a surprising development is consistent with the ironic nature of this song. It does seem unfair for the innocent to die for the guilty. But what is God to do when all have sinned and wandered off like stray sheep (cf. v. 6)? Covenant law demands punishment, but punishment in this case would mean annihilation of what God has created. God’s justice, as demanded by the law, must be satisfied. To satisfy his justice, he does something seemingly unjust. He punishes his sinless servant, the only one who has not strayed off! In the progress of biblical revelation, we discover that the sinless servant is really God in the flesh, who offers himself because he is committed to the world he has created. If his justice can only be satisfied if he himself endures the punishment, then so be it. What appears to be an act of injustice is really love satisfying the demands of justice!
Furthermore, verse 12 of Isaiah seems to provide some additional context for verse 11 which is not so obvious in the KJV but becomes apparent in other versions:
Having suffered, he will reflect on his work, he will be satisfied when he understands what he has done. “My servant will acquit many, for he carried their sins. So I will assign him a portion with the multitudes, he will divide the spoils of victory with the powerful, because he willingly submitted to death
and was numbered with the rebels, when he lifted up the sin of many and intervened on behalf of the rebels.”
So, instead of two separate statements:
1) by his knowledge shall my righteous servant justify many" and 2) he willingly submitted to death ... when he lifted up the sin of many and intervened on behalf of the rebels.
These statements become related in the NET. The servant will acquit many, for he carried their sins and when he willingly submitted to death, he intervened on behalf of the rebels; lifting up the sins of many.
This tends to have a better flow of thought.
Who is the servant?
According to Dr. John D.W. Watts,2
The travail of his soul refers to the suffering and death of Zerubbabel. He will see; he will be satisfied. This speaks of Darius. He has a way out of his dilemma if he treats Zerubbabel’s death as atonement for the charge of rebellion. By knowing about him (Zerubbabel), he (Darius) can justify. The death of Zerubbabel provides Darius with a legal way to resolve the issue. My servant refers to Darius, who by this act proves his legitimacy as Yahweh’s servant. He vindicates Jerusalem and its people against the charges brought by the governor and neighboring peoples. He forgives their wrongs. This is presented as Yahweh’s realistic and practical solution to the problem posed in 52:14–15.
Watts goes on to say,
The arrangement adopted by this commentary places this passage in a specific historical setting. But from that specific setting emerges a universal truth about God and his ways that is vital for the faith of Jew and Christian: the principle of substitutionary atonement, not only through animal sacrifice as in the day of atonement, but supremely through a willing person. This is effective atonement when the recipients of the benefits gained through the sacrifice confess their guilt and recognize that one has died for them (53:4–6) and when the sovereign agrees to recognize the atoning effect (53:10–12).
Both Jewish and Christian tradition regard this passage as also prophetic however, under the principle of double-fulfillment. While Rabbinic tradition (Rashi and later Rabbis for example) regards the servant as the nation of Israel this interpretation seems unlikely. In Isaiah 53:12, we see that the sins of the Rebels are forgiven by the servant. It is impossible for the nation of Israel to be both the persecuted servant and also those in rebellion, be they rebelling against Yahweh, or against the occupying empire (the Romans, Assyrians or Babylonians). The people of Israel cannot be Darius, Zerubbabel and the people of Jerusalem - only the People of Jerusalem. Clearly then, the servant must be the prophesied Messiah. Perhaps it is for this reason that the Talmud and Midrash interpreted the servant of Isaiah 53 as referring to the prophesied Messiah. For this reason, earlier Rabbinic interpretation should be preferred.
What kind of "knowledge"?
The translation of Isaiah 53:11 in the Septuagint is also interesting:
ἀπὸ τοῦ πόνου τῆς ψυχῆς αὐτοῦ δεῖξαι αὐτῷ φῶς καὶ πλάσαι τῇ συνέσει δικαιῶσαι δίκαιον εὖ δουλεύοντα πολλοῖς καὶ τὰς ἁμαρτίας αὐτῶν αὐτὸς ἀνοίσει
σύνεσις is used 7 times in the new testament, but appears 70 times in the Old Testament; mostly concentrated in Psalms and Proverbs. While know or knowledge appears on the order of hundreds of times, it is not always translated from σύνεσις. This tends to indicate that σύνεσις means something more than simply knowing or knowledge. On the other hand, Paul seems to be directly referencing Isaiah 53:11, yet uses γινώσκω instead of σύνεσις in Galatians 4:8-9 (quoted above), so clearly the meanings are somewhat interchangeable and synonymous.
According to the BDAG1, σύνεσις means:
The faculty of comprehension, intelligence, acuteness, shrewdness
the content of understanding or comprehension, insight, understanding
in the religio-ethical realm: understanding such as God grants to God’s own
Knowledge or understanding of the divine mystery
Often "understanding" is a more apt translation of σύνεσις (as the NET does). Paul uses it this way occasionally and partially regards the understanding of the divine mystery (Christ's Sacrifice) as being salvific:
My goal is that their hearts, having been knit together in love, may be encouraged, and that they may have all the riches that assurance brings in their understanding of the knowledge of the mystery of God, namely, Christ,
When reading this, you will be able to understand my insight into this secret of Christ.
(also see Matthew 13:11)
Spiritual knowledge and wisdom
But Paul also believes that there is a spiritual knowledge and wisdom to be imparted to or grasped by believers, which is consistent with the widespread Septuagintal usage of σύνεσις in Psalms and Proverbs:
For this reason we also, from the day we heard about you, have not ceased praying for you and asking God to fill you with the knowledge of his will in all spiritual wisdom and understanding
Knowledge as obedience
The author of 1 John states in 2:3-4 that knowing god can be in our keeping of commandments:
Now by this we know that we have come to know God: if we keep his commandments. The one who says “I have come to know God” and yet does not keep his commandments is a liar, and the truth is not in such a person.
This is true for both Jewish and Christian readers; whether the Messiah is God and we are keeping his commandments, or the Messiah will be a prophet and we keep the commandments that the prophet is giving to his People on behalf of the Almighty.
Intimate, relational knowledge
John's usage in 1 John 2:3-4, as well as Paul's in Galatians 4:8-9 and 2 Timothy 2:19 seem to harken back to John 10:14-16:
I am the good shepherd. I know my own and my own know me— just as the Father knows me and I know the Father—and I lay down my life for the sheep. I have other sheep that do not come from this sheepfold. I must bring them too, and they will listen to my voice, so that there will be one flock and one shepherd.
Again, supporting the interpretation that the salvation or ransom in Isaiah 53:11 is dual: Through both the Messiah's knowing of his sheep and the sheep's knowledge of the shepherd. It hints at a personal and reciprocal relational knowledge.
Knowledge in Christian usage
In addition to the above interpretations, Christian tradition regards the saving knowledge of Isaiah 53:11 as knowledge of Jesus Christ Specifically - Understanding of his death and resurrection; personal, relational knowledge of Christ, obedience to Christ's teachings and spiritual knowledge and wisdom about Christ. Luke 1:77 states this explicitly, saying John the Baptist would:
give his people knowledge of salvation through the forgiveness of their sins.
Jesus arguably references Isaiah 53:11 in Luke 8:16-17, saying:
No one lights a lamp and then covers it with a jar or puts it under a bed, but puts it on a lampstand so that those who come in can see the light. For nothing is hidden that will not be revealed, and nothing concealed that will not be made known and brought to light.
This may be a reference to the light in Isaiah 53:11 ("he will see the light of life and be satisfied") and knowledge of this light reveals the hidden things and mysteries, allowing believers to be brought to the Light.
Teaching the knowledge and understanding of the Light can be seen in the activities of Jesus ministry:
But so that you may know that the Son of Man has authority on earth to forgive sins”—then he said to the paralytic—“Stand up, take your stretcher, and go home.
And in the Evangelism by the Apostles
You know the message he sent to the people of Israel, proclaiming the good news of peace through Jesus Christ (he is Lord of all)
1 Corinthians 1:21
For since in the wisdom of God the world by its wisdom did not know God, God was pleased to save those who believe by the foolishness of preaching.
Peter's confession in John 6:67-69 makes it clear that in the words of Christ is salvation and eternal life:
So Jesus said to the twelve, “You don’t want to go away too, do you?” Simon Peter answered him, “Lord, to whom would we go? You have the words of eternal life. We have come to believe and to know that you are the Holy One of God!”
Which Jesus confirms in John 8:19
Then they began asking him, “Who is your father?” Jesus answered, “You do not know either me or my Father. If you knew me you would know my Father too.”
The servant in Isaiah is Darius and the Messiah. The "knowledge/understanding" is mutual, possessed by both the servant and those in rebellion which lead to rebellion. This view is supported by Dr. Thomas L. Constable who writes in his Notes on Isaiah (also available with the NET)
After His sacrificial work had ended, the Servant would look back on it with satisfaction, as would Yahweh (cf. 1 John 2:2). The many would obtain justification through the knowledge of Him and His work. The “many” is a distinct group, numerous but not all-inclusive, namely, believers. No other work is required but believing what one comes to know, namely, to rely on Him and His work. It is possible that Isaiah meant that the Servant alone would possess knowledge regarding what God required in relation to sin and what He should do about that, but this seems unlikely. One scholar argued that it was the Servant’s knowledge of God and of God’s unfolding purpose for the peoples of the world that satisfied Him and ultimately made many righteous.,” in Israelite Wisdom: Theological and Literary Essays in Honor of Samuel Terrien, pp. 129, 135.
This knowledge is multifaceted and requires spiritual understanding by the mind, wisdom and obedience and a relationship with the servant. Regardless of whether the servant is the messiah or Israel, what is clear is that understanding, knowledge, obedience, wisdom and relationship with God are required of God's people.
1 Arndt, William ; Danker, Frederick W. ; Bauer, Walter: A Greek-English Lexicon of the New Testament and Other Early Christian Literature. 3rd ed. Chicago : University of Chicago Press, 2000, S. 970
2 Watts, John D. W.: Word Biblical Commentary : Isaiah 34-66. Dallas : Word, Incorporated, 2002 (Word Biblical Commentary 25), S. 232