Therefore you shall be perfect, just as your Father in heaven is perfect. (Matthew 5:28)
Adam Clarke: Be ye therefore perfect – as your Father – God himself is the grand law, sole giver, and only pattern of the perfection which he recommends to his children. The words are very emphatic, εσεσθε ουν υμεις τελειοι, Ye shall be therefore perfect – ye shall be filled with the spirit of that God whose name is Mercy, and whose nature is love. God has many imitators of his power, independence, justice, etc., but few of his love, condescension, and kindness. He calls himself Love, to teach us that in this consists that perfection, the attainment of which he has made both our duty and privilege: for these words of our Lord include both a command and a promise.
“Can we be fully saved from sin in this world?” is an important question, to which this text gives a satisfactory answer: “Ye shall be perfect, as your Father, who is in heaven, is perfect.” – As in his infinite nature there is no sin, nothing but goodness and love, so in your finite nature there shall dwell no sin, for the law of the spirit of life in Christ Jesus shall make you free from the law of sin and death (Romans8:2). God shall live in, fill, and rule your hearts; and, in what He fills and influences, neither Satan nor sin can have any part. If men, slighting their own mercies, cry out, This is impossible! – whom does this arguing reprove – God, who, on this ground, has given a command, the fulfillment of which is impossible. “But who can bring a clean out of an unclean thing?” God Almighty – and, however inveterate the disease of sin may be, the grace of the Lord Jesus can fully cure it; and who will say, that he who laid down his life for our souls will not use his power completely to effect that salvation which he has died to procure. “But where is the person thus saved?” Wherever he is found who loves God with all his heart, soul, mind, and strength, and his neighbor as himself; and, for the honor of Christianity and its Author, may we not hope there are many such in the Church of God, not known indeed by any profession of this kind which they make, but by a surer testimony, that of uniformly holy tempers, piety to God, and beneficence to man?