Jesus said to him, “He who is bathed needs only to wash his feet, but is completely clean; and you are clean, but not all of you.”
John 13:10 NKJV
In the OT and NT time, there were no concrete roads. Everything is dusty. So walking makes one's feet dirty. In effect, if one has already taken a bath for the day, he needs only to wash his feet from the dirt it may have accumulated.
Likewise, when Jesus saved us, we have been cleaned. We have been changed. Yet, there are tines when we commit mistakes unwillingly, or at times, even knowingly. For this, we still ask God for forgiveness. It's as if our salvation is our bathe, and asking for forgiveness for those sins we may have had afterwards is our washing of feet.
Jesus made salvation possible for us. Jesus is our doorway to forgiveness, should we sin as we walk our Christian walk.
However, this should not make us deal with sin lightly (no to i-will-sin-now-anyway-i-can-always-ask-for-forgiveness attitude). It should make us all the more careful in our daily walk, because it is still possible to commit mistakes.