The Bible
The Sermon on the Mount presents a radical ethical framework for those living in God's kingdom, contrasting starkly with natural human instincts and conventional morality.
The phrase kingdom of heaven emphasizes a stark contrast between God's radical ways and normal earthly expectations.
Christ did not abolish the Old Testament law but fulfilled it by demanding a deeper righteousness rooted in the heart.
The call to be perfect means striving for a mature and unconditional love rather than achieving a state of absolute sinlessness.