[He that believeth on him is not condemned] His sins being forgiven as a result of his believing in Christ.

[But he that believeth not] The gospel and the way of salvation is plainly understandable to all who seek God. Those who do not seek or do not believe are by default unbelievers.

[Is condemned already] Continues under the condemnation which Divine justice has passed upon all sinners; and has superadded, "He hath not believed in the name of the only begotten Son of God," and is therefore guilty of the grossest insult to the Divine majesty, in neglecting, slighting and despising the salvation which the infinite mercy of God has provided for him.

19 And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
20 For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.

[This is the condemnation] That is, this is the reason why any shall be found finally to perish, not that they came into the world with a perverted and corrupt nature, which is true; nor that they lived many years in the practice of sin, which is also true; but because they refused to receive the salvation which God sent to them.

[Light is come] That is, Jesus, the Sun of righteousness, the fountain of light and life; diffusing his benign influences everywhere and favoring men with a clear and full revelation of the Divine will.

[Men loved darkness] Have preferred sin to holiness, Belial to Christ, hell to heaven.

[Because their deeds were evil.] An allusion to robbers and other evil-doers who practice their abominations in the night season for fear of being detected. The sun is a common blessing to the human race, it shines to all, envies none, and calls all to necessary labor. As for the ungodly, there is light - they refuse to walk in it.

21 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.

[Wrought in God.] In his presence and through his assistance. This is the end of our Lord's discourse to Nicodemus. Though we are not informed here, we learn from other scriptures that it had produced the most blessed effect. From this time he became a disciple of Christ. He publicly defended our Lord in the Sanhedren, of which he was probably a member, John 7:50, and, with Joseph of Arimathea, gave him an honorable funeral, John 19:39, when all Jesus' friends had deserted him.

22 After these things came Jesus and his disciples into the land of Judaea; and there he tarried with them, and baptized.

[Came...into the land of Judaea] Jerusalem itself, where Christ held the preceeding discourse with Nicodemus, was in Judaea, but the evangelist means that our Lord left the city and it's suburbs and went into the countryside.

23 And John also was baptizing in Aenon near to Salim, because there was much water there: and they came, and were baptized.
24 For John was not yet cast into prison.

[In Aenon] This place was eight miles southward from Scythopolis, between Salim and Jordan.

[There was much water] And this was equally necessary where such multitudes were baptized. The Jewish custom required the persons to stand in the water, and, having been instructed, entered into a covenant to renounce idolatry and take the God of Israel for their God, then plunge themselves under the water.

[John's disciples and the Jews] A debate, possibly about whose baptism was more effective.

26 And they came unto John, and said unto him, Rabbi, he that was with thee beyond Jordan, to whom thou barest witness, behold, the same baptizeth, and all men come to him.

[all men come to him] Some of John the Baptist's diciples remained with him even after he had pointed out Jesus as the Christ.

27 John answered and said, A man can receive nothing, except it be given him from heaven.

[Except it be given him from heaven] I have received, not only my commission, but the power also by which I executed it. As I took it up at God's commandment, so am I ready to lay it down when he pleases. I have told you from the beginning that I was only the forerunner of the Messiah; and was sent, not to form a separate party, but to point men toward that Lamb of God which takes away the sin of the world.

28 Ye yourselves bear me witness, that I said, I am not the Christ, but that I am sent before him.
29 He that hath the bride is the bridegroom: but the friend of the bridegroom, which standeth and heareth him, rejoiceth greatly because of the bridegroom's voice: this my joy therefore is fulfilled.
30 He must increase, but I must decrease.

[He must increase] His present success is but the beginning of a most glorious and universal spread of righteousness, peace, truth and good will among men.

[I must decrease] My baptism and teaching, as pointing out the coming Messiah, must cease because the Messiah is now come and has entered publicly on the work of his glorious ministry.

31 He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all.

[Is above all] This blessed bridegroom, who has descended from heaven, is above all, superior to Moses, the prophets and me.

32 And what he hath seen and heard, that he testifieth; and no man receiveth his testimony.
33 He that hath received his testimony hath set to his seal that God is true.
34 For he whom God hath sent speaketh the words of God: for God giveth not the Spirit by measure unto him.

John here explains that he received his assignment and unique testimony from the Holy Spirit and that he, "He that hath received his testimony," has affirmed it to be true and has been speaking the very words given him by God.

35 The Father loveth the Son, and hath given all things into his hand.

[All things into his hand.] A principle design of John is to show that Christ is infinitely above every teacher, prophet or Divine messenger that has ever appeared. Christ, with the blessing of the Father, has chosen to redeem sinfull mankind by his substitutionary death on the cross.

36 He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him.