Until the fashion came about to rank all Sundays ahead of major feast days, today would have been celebrated as the Feast of St. John the Apostle and Evangelist. As it is, he now gets transferred to a later day in the week.[1] But still he has a voice today, doing what he does best, proclaiming the news of the Kingdom of God as shown in Jesus Christ.
Today's Gospel at Mass is taken from the Prologue of the Gospel attributed to St. John. This Gospel is at once the easiest and most difficult to comprehend, simple yet complex in meaning, immediately approachable but deep with complex theological themes. Among the Orthodox this Gospel was considered the one which fed the newly baptized, unlike the others which were for the edification of catechumens preparing for baptism.
In this Prologue, we hear that the Eternal Word of God, through Whom all things were made, Who was the Light, became flesh and dwelt among us. This light shines in the darkness. The darkness does not understand it. The darkness wants to snuff it out. But the darkness could not, can not, will not overcome it. This Light is Jesus Christ, full of grace and truth. The apostle John wants us to understand this. St. John takes pains to explain to us that Jesus wasn't just some prophet, some rabbi, some extraordinarily great preacher and teacher. No, this man was also God Who had taken on our humanity in order to bring us His Light, the Light the Darkness to which we were enslaved cannot overcome. He proclaimed the Kingdom of God, and thanks to St. John's witness (among others), that Kingdom is still proclaimed, and according to St. John's witness (among others), the Darkness that enslaves us has been broken, it has failed in its goal to overwhelm the Light, and from which Jesus has redeemed us (at considerable cost).
The rest of the struggle is learning to put off the darkness, to unlearn being a slave, and to rejoice, having turned back to the Light. Thanks be to God.
St John the Theologian “in Silence” (Village of Vladimir, 18th Century)
[1] The Orthodox don't have this problem as they celebrate St. John's feast on May the 8th. A write-up on their perspective (and the icon type illustrated here) can be found at "A Readers Guide to Orthodox Icons" https://iconreader.wordpress.com/2012/05/07/john-the-apostle-the-theologian-in-silence/