As I've read the Sermon on the Mount I have been amazed at one consistent quality in the sermon. Jesus pushes his disciples beyond what human limits say is possible. The Law of Moses, which they had been raised on, said "Do not murder (Ex 20:13)". Not murdering is pretty easy for most of us. But Jesus raises the ante on his followers, "I tell you if anyone is angry with a brother he will be subject to judgment". Now I have a brother and a sister, both younger, and I know that I've been angry with them for many different reasons (some valid, some not so much)...yet I claim to be a follower of Christ. He goes on, "if any one lusts" he's (or she's) already committed adultery. And again, "if you remember a brother has something against you, leave your gift in front of the altar and go be reconciled". What Jesus expects is impossible....or is it.
I've sat through three days of my forgiveness in counseling class and I've come to one conclusion. Everything is a matter of our source. If we try to forgive our brothers and sisters so we can be reconciled the way Jesus commands (and it is a command not a request or a suggestion) in our own power we will undoubtedly fail. If we try to control our anger, our lust, our thirst for vengeance... we simply cannot do it. We are fallen creatures living in a fallen world, even after we have been redeemed the effects of the Fall still limit our abilities.
The kicker is this...if we root ourselves in the Triune God then we can avail ourselves of the power He offers. By the transforming power of the Holy Spirit we become the "blessed" people that Jesus refers to in Matt 5:3-12. This radical, life-changing Spirit is only available because of the work of Jesus Christ. He did perfectly what he teaches his disciples to do....ultimately it boils down to obeying the Father. The Sermon on the Mount just neatly lays out what a life lived in glory to the Triune God will look like and the Lord's Prayer shows us how we can actually live it.