“friend of the bridegroom” is similar to our “best man.” The friend of the bridegroom was responsible for many of the details of the wedding and in particular (and most importantly) for bringing the bride to the groom.  But after he had brought her forth to the groom, his job was over and he was just another participant in the joy of the groom at the wedding.  The important man at the wedding was the bridegroom, not the friend of the bridegroom.

In the Old Testament Israel is called the bride of YHWH.

Isaiah 54:5  For your Maker is your husband—the LORD Almighty is his name—the Holy One of Israel is your Redeemer; he is called the God of all the earth.

Hosea 2:19-20  I will betroth you to me for ever; I will betroth you in righteousness and justice, in love and compassion. I will betroth you in faithfulness, and you will acknowledge the LORD.

Christ was also prophesied as being a bridegroom (Psalms 45).

1 For the director of music. To the tune of “Lilies”. Of the Sons of Korah. A maskil. A wedding song. My heart is stirred by a noble theme as I recite my verses for the king; my tongue is the pen of a skilful writer. 2  You are the most excellent of men and your lips have been anointed with grace, since God has blessed you for ever. 3  Gird your sword upon your side, O mighty one; clothe yourself with splendour and majesty. 4  In your majesty ride forth victoriously on behalf of truth, humility and righteousness; let your right hand display awesome deeds. 5  Let your sharp arrows pierce the hearts of the king’s enemies; let the nations fall beneath your feet.

6  Your throne, O God, will last for ever and ever; a sceptre of justice will be the sceptre of your kingdom. 7  You love righteousness and hate wickedness; therefore God, your God, has set you above your companions by anointing you with the oil of joy. 8  All your robes are fragrant with myrrh and aloes and cassia; from palaces adorned with ivory the music of the strings makes you glad. 9  Daughters of kings are among your honoured women; at your right hand is the royal bride in gold of Ophir.

Matthew Henry observes that “The Word was made flesh, that the disparity of nature might not be a bar to the match. Provision is made for the purifying of the church, that the defilement of sin might be no bar.”

As Murray points out, “In some real sense the Baptist testified that God Himself was in Christ betrothing His bride to Himself afresh.” (qt’d in Morris p.241) John’s task was to bring Israel, the bride, to Jesus, the bridegroom. After he had accomplished this, his job was over.