It was in the year of King Uzziah's death (a great and good king, but who unfortunately was punished by God with leprosy for offhandedly not respecting the holiness of the temple worship), that the prophet Isaiah, who was close to the King and may have been dismayed by his divine punishment, had a vision of God in the temple surrounded by angels who cried out to one another, "Holy, holy, holy is the Lord God". And then Isaiah became aware of his own sinfulness too in the presence of such a holy God, "I am lost, for my eyes have looked at Yahweh, the King, I, a wretched man of unclean lips." Then one of the seraph angels took a burning coal from the altar and touched the mouth of Isaiah saying, "Now your sin is taken away and your iniquity is purged" (Is 6:1-7). This experience had made such a great impact on Isaiah that in his prophecy he constantly refers to God as the Holy One, more than any other book of the Bible does. He must have called to mind then what God says in Leviticus, the OT book on holiness, "The LORD said to Israel through Moses, 'I am the LORD your God. I am holy, and you must be holy too.'" (Lev 19:2).
Be holy as I am holy
God is holy:
This Holy God therefore speaks his holy word (Jer 23:9), he remembers his holy promise (Ps 105:42), he stretches forth his holy arm (Is 52:10), he is jealous of his holy name (Lev 20:3). The place where he appears is holy ground (Ex 3:5), anything set apart for his sacred use is holy, persons connected with these holy places and holy services are holy, and times given to worship are holy (Ex 12:16; 16:23 Is 58:13). Jesus himself at his conception was called the Holy One of God and the devil too recognized and acclaimed him as such, "I know who you are" (Mk 1:24). Jesus even threw a challenge to his detractors, "Can any one of you convict me of sin?" (Jn 8:46), while Pilate could find no case against him in spite of all the accusations made by the religious leaders (Jn 18:38). Hence Jesus addressed God as 'Holy Father' (Jn 17:11) and taught his disciples to pray that the name of their Father God may be held holy and to ask him to give them the Holy Spirit (Lk 11:2,13).
And you must be holy:
God is holy - and Christ died for the Church to make it holy. Hence both his people are to be a holy nation (1 Pet 1:15; 2:9; Eph 5:26), as well as individual Christians, his children, created in his image, are to be called the 'saints' (holy) (Rom 1:7; 1 Cor 1:2), - purified by the Holy Word of God that challenges them (Jn 17:7), and sanctified by the Holy Sacrifice of the Mass that enables them, to acknowledge their sins, not only in deeds but in words and even in thoughts, and not only in what they have done but also in what they have failed to do, while the 'holy kiss', in the early church, was a seal of holy fellowship (1 Cor 16:20). Thousands have thus answered the call of God to become holy, as he is holy, and have lived such holy lives that the Church has even officially recognized them as holy (canonized them). And so as both the one who makes men holy and those who are made holy are of the same family, Jesus is not ashamed to call them openly his brothers (Heb 2:11).
Be perfect as your Father is perfect
While in the Old Testament God the Father calls us to be holy as he is holy, in the New Testament Jesus his Son calls us to be perfect as his and our Father is perfect.
God is perfect:
perfect in his way, in his word, and in his works. He is a faithful God, upright and just, who does no wrong, though in contrast Israel "has acted foolishly and corruptly toward him. He is their Father and Creator, but to their shame they are no longer his children, but a warped and crooked generation" (Deut 32:4-6). Jesus therefore came to show by word and example how his disciples should respond to a perfect God and Father. At the beginning of his ministry, on their urging him to eat, he replied, "My food is to do the will of the one who sent me, and to do it perfectly" (Jn 4:34), and at the end of his life and ministry he could state, "Done" (Jn 19:30).
And you must be perfect:
Once a rich young man came and asked Jesus, "What must I do to have eternal life". "Keep the commandments", he was told. "I have kept all these from my earliest days till now", he answered, "What more do I have to do?" Jesus looked at him with love and said, "There is still one thing you lack. If you wish to be perfect, then go and sell everything you own and give the money to the poor and you will have treasure in heaven; then come, follow me" (Mt 19:14-22). For Jesus made it clear to his disciples, "If your virtue goes no deeper than that of the Scribes and Pharisees, you will never get into the kingdom of heaven" (Mt 5:20). Thousands of men and women have done this down the centuries, following the poverty Gospel rather than the prosperity Gospel.
Again to those whose religious convictions led them to love their neighbor but to hate their enemy, Jesus declared, "I tell you however: Love your enemies and pray for those who persecute you. In this way you will be sons of your Father in heaven, who looks after both bad men as well as good, and after honest and dishonest men alike. For if you love those who love you, what reward can you expect? Are not even the tax collectors doing that? And if you greet only your brothers, are you doing anything exceptional? Do not even pagans do that?" To Jesus the conclusion is obvious; "You must therefore be perfect just as your heavenly Father is perfect" (Mt 5:43-48). Thus Paul's ambition is to be made perfect, though he knows he is far from it (Phil 3:12-14), and to struggle with all his energy to make everyone perfect in Christ. (Col 1:28-29), - so perfect that they are never at fault in what they say (Jas 3:1), and are able to keep their whole body in check and to love so perfectly that there is no more fear (Jas 3:18).
Be compassionate as your Father is compassionate
Thirdly and finally we are called not only to be holy, as Leviticus, the Book of Holiness, enjoins us, by avoiding sin; and to be perfect, as Mathew's Gospel of Perfection commands us, by doing good; but to even imitate God our Father, as Luke's Gospel of Compassion urges us, by being tenderhearted.
God is compassionate:
Already in Exodus, the Old Testament book of God's compassion, God proclaims himself as "The LORD, but a God of tenderness and compassion, slow to anger, rich in kindness and faithfulness, maintaining love to thousands, and forgiving wickedness, rebellion and sin" (Ex 34:6,7). And in Nehemiah, the people themselves admit, "Our forefathers became arrogant and stiff-necked, forgot your wonders, refused to obey you and flouted your commands. But you are a God of forgiveness, gracious and compassionate. Therefore you did not desert them, even when they worshipped an image of a calf as their god and saviour, or when they committed awful blasphemies" (Neh 9:16-18).
And you must be compassionate:
It is in Exodus too that God calls on Israel to be compassionate, as he is compassionate. "If you lend money to any of my people among you who is needy, do not be like a moneylender, but charge him no interest. If you take your neighbor's cloak as a pledge, return it to him by sunset, because his cloak is the only covering he has for his body. What else will he sleep in? And if he cries out to me, I will hear him, for I am compassionate" (Ex 22:25-27). In stark contrast Jonah was very indignant and fell into a rage precisely because God showed mercy to the people of Nineveh. "That is why I was so quick to flee to Tarshish. I knew that you were a God of tenderness and compassion, slow to anger and abounding in love, who relents from sending calamity. Now, O LORD, please take away my life, for it is better for me to die than to live (as you want me to live - full of compassion like you)" (Jon 4:3).
But it is in Luke's Gospel of Compassion that Jesus spells out compassion as the heart of holiness and the pinnacle of perfection. "But I tell you: Love your enemies, do good to those who hate you, bless those who curse you, pray for those who treat you badly. If you love those who love you, what credit is that to you? Even sinners do that. And if you do good to those who are good to you, what credit is that to you? Even sinners do that. And if you lend to those from whom you expect repayment, what thanks can you expect? Even sinners lend to sinners, expecting to be repaid in full. Instead, love your enemies and do good to them, and lend without any hope of return. Then your reward will be great for you will qualify to be sons of the Most High, since he himself is kind to the ungrateful and the wicked. Therefore be compassionate as your Father is compassionate" (Lk 6:27-36).
On the last day God will judge the perfection and even the holiness of our lives by our compassion (Mt 25:31-46). For God is holy, God is perfect, but above all God is compassionate, - in fact God is Love, and He has called us not only to be holy and perfect but to be compassionate, for anyone who lives in love lives in God, and God lives in him (1Jn 4:16).