When Jesus commands us to be perfect as our Father in heaven is perfect (Matt. 5:48), this simply shows that God’s own absolute moral purity is the standard toward which we are to aim and the standard for which God holds us accountable. The fact that we are unable to attain that standard does not mean that it will be lowered; rather, it means that we need God’s grace and forgiveness to overcome our remaining sin. Similarly, when Paul commands the Corinthians to make holiness perfect in the fear of the Lord (2 Cor. 7:1), or prays that God would sanctify the Thessalonians wholly (1 Thess. 5:23), he is pointing to the goal that he desires them to reach. He does not imply that any reach it, but only that this is the high moral standard toward which God wants all believers to aspire.
Wayne Grudem

Bible in One Year