What our eyes feed on is pivotal. It becomes the essence of our soul and has a cumulative effect on every aspect of our spiritual standing. To help us absorb the lofty concepts of purity, the Midrash (Tanchuma, Kedoshim) gives a picturesque parable of a royal wedding day. The groom is a king. He turns to his bride and says, ״Now that we are about to be married to one another, I am King and you are Queen. Henceforth, whatever is my honour is also your honour as today you become my wife.״
When B'nei Yisrael came to Har Sinai, Hashem told Moshe to prepare them to receive the Torah. The word He used, וקדשתם , translates as sanctify them! This word also carries a connotation of ״kiddushin״ - a marriage. Yes, it was a marriage, no less, which happened at Har Sinai. We were elevated to the status of ״mekudeshes,״ becoming the intimate partner of Hashem Himself, so to speak.
And what will serve as the foundation to ensure the compatibility of that marriage? A firm commitment on our part to modesty and purity. Hashem declared, ואתם תהיו לי ממלכת כהנים וגוי קדוש - You will be to Me a Kingdom of priests and a holy nation. Why is it so necessary for us to be a holy nation? The Midrash (ibid.) quotes a second passuk to answer: כי קדוש אני - For I [Hashem] am holy. Our special relationship with Hashem is contingent upon our purity.
Now, flowers may fade, but at Har Sinai an everlasting covenant was forged between Hashem and the Jewish People - a partnership to endure for all time and reaching to the furthest lands of our dispersion. How shall we keep fresh the magic of that glorious day and hold onto the wonder and excitement of His warm embrace? Only by preserving our deep, Jewish, innate feel for modesty and purity, thus cherishing the sweet exclusivity of the relationship between ourselves, a unique and holy nation, and our Holy Creator.
Purity becomes the cornerstone of the Jew's entire spiritual standing and entitles him, in the World to Come, to be called by name to take his place among those distinguished to remain in priceless proximity to Hashem Himself. This person purified his soul, clung fast to Torah and mitzvos, rejected all foreign invasions, and thereby earned for eternity the title ״Beloved Partner of the Creator of the Universe.״
Speaking of eternity, the Chafetz Chaim says succinctly: ״A person's level of closeness to Hashem in Olam Haba is determined by the amount of kedusha that his soul absorbed by doing mitzvos in This World״ (Torah Ohr, ch. 7).
The Vilna Gaon, ztz״l, comments that the passuk (Bereishis 38:21), איה הקדשה היא בעינים can be interpreted as, ״All of a person's holiness depends on his eyes״ It follows that if you want to become holy, then start with keeping your eyes pure.
The Ra'avad condenses our topic into a few sentences:
A person's first line of defense against sin is his shmiras einayim.
If his eyes are guarded then his mind is guarded.
If his eyes and mind are guarded, then he is completely guarded!