The L-rd make His face shine upon you...the L-rd lift up His countenance to you (Num. 6:25-26)
It is written in the holy Zohar that the letters of G-d's Name engraved on the golden plate on the High Priest's headdress were luminous. Anyone looking at them was filled with awe; this created an arousal to return to G-d in repentance, and the person's sins would be atoned for. In other words, through the luminous letters ("the L-rd make His face shine") the Jews repented (allowing G-d's countenance to be "lifted up"), and their sins were forgiven. (Kotnot Or)