Because I'm a missionary, this concept reminds me of a scripture. There's a part in The Book Of Mormon when a bunch of Lamanites (usually the bad guys) become converted to the Lord and join God's church. The king of this people expresses their feelings about repentance: "it was all we could do to repent sufficiently before God that he would take away our stain" (Alma 24:11). When we make mistakes, or sin, it effects us tremendously. We have feelings of guilt and shame. These feelings put a "stain" on our hearts. The only way to get rid of this stain is to repent and ask God for forgiveness. When removed, we don't need to worry about hiding our sins or our past transgressions from others. All of those bad feelings we have will be gone. Just like that really neat pen takes the stain from my shirt, through repentance, God will take the stain from our hearts.
While this awesome pen can quickly and easily solve many problems with my white shirts, it can't do everything. Some stains, because of the size and severity of the stain, need further treatment. This treatment usually takes a lot more work than the convenient pen. Without extensive care, the shirt could be lost forever. It's not as easy but it's just as important. It's the same way with sins. Repentance isn't always the easiest process. Just like the huge stain on my shirt, a very serious sin can only be removed by much prayer and forgiveness.