This is perhaps one of the most misunderstood and misused passages in the Sermon on the Mount. By effectively treating it in isolation from a careful consideration of the balance of the discourse (not to mention in isolation from Jesus' overall teaching in the Gospels), many Christians interpret "doing the will of the Father" as a call to strict obedience to commanded behaviors. This perception is compounded by people's inclination to read the Scripture through the lens of their own time and culture. Being sensitive to the contemporary Christian culture of easy-believism and situational ethics, it's easy for Christians to find in Jesus' words a stern declaration that only "doers" and not "sayers" will inherit heaven. But, ironically, this idea of finding confidence in "doing" is exactly what Jesus was rebuking and warning about. His Jewish audience wasn't given to "easy-believism"; to the contrary, they were fully convinced that their confidence before God was found in meticulous conformity to the Law of Moses. This was the case for the majority of those who embraced Jesus as Messiah as much as for those who rejected Him as an imposter (John 8:31ff). The Jews' problem wasn't doctrinal and ethical relativism and easy-believism; it was the self-righteousness of strict piety. Thus Jesus' call to "do the will of the Father" wasn't an exhortation to godly conduct in His name; He denounced such "doers" as lawless ones. To do the will of the Father is to believe in the One whom He has sent (John 6:26-29). Finding one's righteousness in Christ - in sanctification as much as justification - is doing God's will and obeying His law (Galatians 3-4).
Great Sermon! All 'believers' please hear this sermon! Their is a radical and eternal difference between piety, commitment and lawful obedience - and faith, hope and love. Yet we all tend to confuse or even equate the two - just as many 'religious' men in Jesus's day (that we might scoff at), like Saul.