In this week's Parsha we read about the Avodah of Yom Kippur. The Posuk states, "ונתן את הקטרת על האש לפני ה'," "and he shall place the incense on the fire before Hashem" (ויקרא ט"ז:י"ג). The Kohein Gadol is told to take a pan of burning coals and a certain amount of incense and to enter the Kodesh HaKodashim, the holiest place in the Beis HaMikdash, and burn the incense. The place where this offering is burnt is the most sacred room of the sanctuary; it is a place where no one is ever to enter at any other time, no matter who that person is, and it is in that place that the sins of the people are atoned for.
The Chofetz Chaim writes that we should think about this fact that our sins have unfortunately reached this most holy place and thus need to be atoned for there. He notes that the Gemara in Yoma (דף מ"ד.) states that the incense offering brings atonement particularly for the sins committed with one's tongue, and one should thus feel distraught about the fact that his sinful use of his tongue has made an impression even in the Kodesh HaKodashim. Similarly, we find in Tanna DeBei Eliyahu (פרק י"ח) that Lashon Hora reaches the Divine throne of glory.
It is also important to realize that the Kohein Gadol enters the Kodesh HaKoDashim only this once a year and his first act of Avodah there is the burning of the Ketores, which brings about the atonement for the sins of the tongue; the Gemara in Yoma (שם) states that this hidden and unseen Avodah brings atonement for the hidden and unseen sin of Lashon Hora which is spoken in private. From the fact that this is the very first sin atoned for, we see how great the damage of Lashon Hora is, since it reaches even to the Kodesh HaKodashim, the innermost sanctuary of Hashem's Mikdash. The Kohein Gadol thus must deal with this sin as soon as he is in there. The Kohein Gadol could approach Hashem to ask forgiveness for all of the people's other sins only after taking care of Lashon Hora; Lashon Hora thus had the capacity to hold back the atonement of all other sins because without properly offering the Ketores, which atoned for Lashon Hora, the Kohein Gadol would be deserving of death. We may thus learn that when any person tries to straighten himself out spiritually, he should first take care of eliminating his Lashon Hora.