"For he whom God hath sent speaketh the words of God: for he giveth not the Spirit by measure."
To understand this passage, we must determine to whom the pronoun refers in the phrase "he whom God hath sent." The most common idea is that it is Jesus. It is said that God gave him the spirit "without measure," implying that God gave the spirit "with measure" to others. Jesus could "do it all" by the Holy Spirit. The apostles, on the other hand, received the "baptismal measure," etc. Others, through the laying on of hands, received the "speaking in tongues measure," or the "prophesying measure," etc. However, that position requires several assumptions I do not believe are found in the context. It begins with John 1:6-8:
"There came a man, sent from God whose name was John. The same came for witness, that he might bear witness of the light, that all might believe through him. He was not the light, but came that he might bear witness of the light."  
We are all familiar with John the Baptist's coming by prophesy as the forerunner, announcer, and preparer of the way for Jesus, Matthew 3:1-12.  The above passages show that he was sent with a commission to act as an agent of God, witnessing and testifying that Jesus was the Son of God.  A consistent and well known fact of Bible teaching is that more than one witness is required in determining the truth of some matter.  In John 5:31, Jesus says,  "If I bear witness of myself, my witness is not true." The testimony of Jesus alone was not enough. This is seen again in John 8:13, "The Pharisees therefore said unto him, Thou bearest witness of thyself; thy witness is not true."  A second witness was needed.
The witness of John the Baptist was temporary, so the other witnesses testified that Jesus came from the Father, John 5:30-47.  Namely, these were the writings of Moses as well as the miraculous works Jesus performed.  However, for that short time, John was the second witness to the person of Jesus.  In John 5:33, Jesus says, "Ye have sent unto John, and he hath born witness unto the truth." This refers us back to the event recorded in John l:19ff.  There, through verse 34, John testifies as a witness to whom Jesus really was. What John spoke was revelation from God by the Holy Spirit.  John 3:26-30 describes John's disciples asking him about Jesus "to whom thou hast born witness." John testifies again about the Lord's being Christ. John then says, vss. 30-31,
"he that cometh from heaven is above all. What he hath seen and heard, of that he beareth witness; and no man receiveth his witness."
Jesus bore witness of himself, testifying to what he had seen and heard in heaven.  Notice the contrast between the way Jesus could testify and the way others had to, such as John.  John spoke by revelation what he was told to say. Jesus witnessed to what he had seen and heard, a matter of experience remembered.  In verse 27, John says that
"A man can receive nothing, except it have been given him from heaven."  
That's the way it is with men. However, Jesus came from heaven and is over all. He testifies to what he has seen and heard.
John said in verse 32 that "no man receiveth his witness." In the next verse he adds, "He that hath received his witness hath set his seal to this, that God is true." John is speaking of himself.  He is speaking in the same sense of "receiving" as in verse 27, which says that a man can "receive" nothing unless it is revealed to him from heaven.  Again, John spoke by revelation, it was "given him from heaven."  This brings us to verse 34:
"For he whom God hath sent speaketh the words of God: for he giveth not the Spirit by measure."
The word "for" introduces a reason for the preceding statement. The person in verse 33 who has "received his witness" and set his seal to it, is the same person in verse 34 that is "he whom God hath sent" that "speaketh the words of God." The subject in both passages is John.  He was sent of God, John 1:6-8.
John "received" his information from God, and so can "speak the words of God." Being an inspired man, John could "set his seal" to that witness to certify it was true, so there would be two witnesses. This is exactly what John did, John 1:19-34, 3:26-30. Again, notice the difference drawn between what it says about Jesus, and then John.  In verse 33, it says that the one from above, Jesus, witnessed to what "he had seen and heard." The person in verse 34, spoke by revelation through the Holy Spirit. By context, verse 34 could not refer to Jesus.
So, John the Baptist is "he whom God hath sent" who "speaketh the words of God" in verse 34. However, the one "whom God hath sent," in verse 34, is the one who was given "the Spirit without measure." Therefore, John the Baptist was the one who received the Spirit without measure.  Notice that when it says "for" he giveth not the Spirit by measure, it is referring to not giving the spirit by measure in speaking the words of God.
All that the phrase means is that John was given by revelation everything he needed to know to do the job he was given to do.  We might say it as "God does not give the Spirit sparingly, or with insufficient measure." John could fully and completely testify as a witness, speaking the words of God, because God does not do things half-way.  This fact was true anytime the Holy Spirit did anything. The Spirit was not given in any insufficient way to the Apostles in revelation, working of miracles, or anything else that was done.
Someone in the past came up with the conclusion that John 3:34 referred to Jesus. Therefore, it refers to Jesus' having all the power the Holy Spirit could give, while the Apostles and others received the Spirit "by measure."  So, we are to refer to "the Baptismal measure" of the Holy Spirit, or "the spiritual gifts measure," etc. However, all of that is asserted from a supposed implication based on an interpretation of John 3:34.
Someone may be quick to invoke I Corinthians 13: "We know in part and we prophesy in part; but when that which is perfect is come, that which is in part shall be done away."  However, this only refers to spiritual gifts that were temporary in nature, given until the New Testament could be collected in its final form. Then the gifts would cease. It does not imply any such thing as "measures" of the Spirit, nor any insufficiency in the working of the Spirit.  He gave them everything necessary to the task at hand.  
Neither do the "divers portions and in divers manners" of Hebrews 1:1 have anything to do with "measures of the Spirit." That only refers to the giving of revelation to different people, at different times, in different ways.  We might as well speak of the "visions measure," the "dreams measure," the "still, small voice measure," the "handwriting on the wall measure," or the "appearance of an angel measure" of imparting revelation from God.  That would be "different measures" within just revelation alone, but that idea isn't even involved.
Whatever else can be said about the different ways the Spirit has worked, John the Baptist was specifically sent from God to bear witness to Jesus by revelation through the Holy Spirit. John received everything he needed in order to do what he was sent to do.  That is what John 3:34 is telling us.
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