He gave them Authority

Matthew 10:1-4 (ESV)
And he called to him his twelve disciples and gave them authority over unclean spirits, to cast them out, and to heal every disease and every affliction. [2] The names of the twelve apostles are these: first, Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother; [3] Philip and Bartholomew; Thomas and Matthew the tax collector; James the son of Alphaeus, and Thaddaeus; [4] Simon the Cananaean, and Judas Iscariot, who betrayed him.

Gave them authority, it was his authority to give. It is often with wonder I contemplate those who deny that someone other than Christ can have this authority, which is none other than the forgiveness of sins, and those who also would usurp for themselves this same authority. It is ironically often the same people who do both in one way or another.
So often you hear people with little theological traing, or understanding of the gospel, who will say that a man cannot forgive sins. This in contrast to the very fact that God has commanded us to forgive sins, even lay men and women are told to forgive those who trespass against them. With the command comes the authority to do so. When a commanding officer orders a soldier to do something, he thereby gives the soldier the authority to do so. Assuming the commanding officer had the authority to issue the order in the first place. So to question whether a pastor or a Christian has the authority to forgive sins, is in reality to question whether Christ had the authority to forgive sins.
Forgive sins, that is what is at issue, even when Christ heals without uttering the word’s your sins are forgiven, it is by the forgiveness of sins that the healing is done. This is because sin is the cause of these things in the first place. Not as if there are corresponding sins for corresponding illnesses and ailments, though some might be closely linked where stds are concerned. However even there one must be careful, because many a faithful spouse has contracted an std from an adulterous partner. But sin, is at the root cause of all disease, just as death is the wages of sin, and death is the result of sickness and disease. So where healing has taken place, where demons have been repelled there sins have been forgiven, there death has had to retreat. Sin affects body and soul. So where the authority to heal has been given, there also has been given the authority to forgive sins.
But this is not something that can just be usurped as the seven sons of Sceva were wont to learn. Yet it is often just those who themselves would question the authority of Christ, who would themselves try to usurp the office that Christ has entrusted with the authority to forgive. This was at the root of the seven sons of sceva, they did not believe in Jesus Christ, but wanted to use his name as a magic talisman. Perhaps not everyone who does this gets the same lesson that the sons of Sceva did. It would be nice if that were the case.
Others set up without the blessing of a congreagation fellow believers, without the blessing fo the church, and begin to teach all sorts of hideous things. Where the authority to forgive sins is questioned, there cannot but be heresy taught. There it is not pure hearts and good consciences that are delivered, which issue forth love, but it is law that is taught without understanding.