17 And the seventy returned again with joy, saying, Lord, even the devils are subject unto us through thy name.
18 And he said unto them, I beheld Satan as lightning fall from heaven.
19 Behold, I give unto you power to tread on serpents and scorpions, and over all the power of the enemy: and nothing shall by any means hurt you.
20 Notwithstanding in this rejoice not, that the spirits are subject unto you; but rather rejoice, because your names are written in heaven.
-The disciples came to Jesus and were very joyous over what they had seen and done. Jesus explains to them why they have such power, but is quick to tell them that they should not joy about it for the day may come, as it did in Matthew 17:15-18, when they may not be able to cast out a devil. If our joy is tied to any circumstance, no matter how God-honoring, then we are in danger of losing our joy when that circumstance changes. Jesus reminds His disciples that the only thing safe to rejoice in is their place in Heaven because that is the one thing that cannot be lost or changed. As the Lord promised in John 6:37, All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.