Context-wise, it makes more sense for Jesus to say "I" because the focus of the passage is completely on him alone and the reason as to why he was sent. Looking at the greater context of the Gospel of John and its theology, it would be erroneous at this point for Jesus to say that anyone other than himself should do the kind of work being referenced. In John 6:28 Jesus was asked, "What shall we do, that we might work the works of God?", to which Jesus replied in John 6:29, "This is the work of God, that ye believe on him whom he hath sent." The work for the rest of the people was to believe on Jesus. It was not for anyone other than Jesus to do the "work" at issue in John 9:4, which was to heal a man born blind so that "the works of God should be made manifest in him" (John 9:3). This uniqueness of Jesus' responsibility to work to reveal the "works of God" is established back in John 5:17-20, 30:
The KJV says "I must do the work..." whereas modern translations tend to say "We must do the work...". The KJV reading is supported by Aleph1 (4th-6th century), A (5th century), C (5th century), Θ (9th century), 33 (9th century), Ψ (9th/10th century), the majority of Byzantine manuscripts, Vulgate and part of the Old Latin tradition, Syriac, Subakhmimic Coptic, some Bohairic, and other witnesses (Nestle-Aland: Novum Testamentum Graece, 27th revised edition (2006)). "We must..." is supported by P66 (3rd century), P75 (3rd century), Aleph* (4th century), B (4th century), D (6th century), L (8th century), W (4th/5th century), 070 (6th century), Sahidic, Proto-Bohairic, Bohairic, and other witnesses. While the KJV reading is supported by later manuscripts, these witnesses are greater in number and come from across greater geographical areas and languages. A single erroneous source in Egypt at an early stage of transmission could account for all of the witnesses going against the KJV reading (See: Aren't older manuscripts more reliable?).
"I must work the works of him that sent me, while it is day: the night cometh, when no man can work." (John 9:4, KJV)
17 But Jesus answered them, My Father worketh hitherto, and I work. 18 Therefore the Jews sought the more to kill him, because he not only had broken the sabbath, but said also that God was his Father, making himself equal with God. 19 Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise. 20 For the Father loveth the Son, and sheweth him all things that himself doeth: and he will shew him greater works than these, that ye may marvel.... 30 I can of mine own self do nothing: as I hear, I judge: and my judgment is just; because I seek not mine own will, but the will of the Father which hath sent me.
In the Gospel of John, whenever "work" is connected to the works of the Father who "sent" the Son, it always refers to the unique work of Jesus Christ.
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