All true holiness comes from God, who is called ‘The Holy One’ in the sacred books of the Jews, Christians, and Muslims. Your holy Qur’an calls God ‘Al-Quddus,’ as in the verse: ‘He is God, besides whom there is no other, the Sovereign, the Holy, the (source of) Peace’ (Qur’an 59, 23). The prophet Hosea links God’s holiness with his forgiving love for mankind, a love which surpasses our ability to comprehend: ‘I am God, not man; I am the Holy One in your midst and have no wish to destroy’ (Ho 11:9). In the Sermon on the Mount, Jesus teaches his disciples that holiness consists in assuming, in our human way, the qualities of God’s own holiness which he has revealed to mankind: ‘Be holy, even as your heavenly Father is holy’ (Mt 5:48).
“Thus the Qur’an calls you to uprightness (al-salah), to conscientious devotion (al-taqwa), to goodness (al-husn), and to virtue (al-birr), which is described as believing in God, giving one’s wealth to the needy, freeing captives, being constant in prayer, keeping one’s word, and being patient in times of suffering, hardship and violence (Qur’an 2:177). Similarly, St. Paul stresses the love we must show toward all, and the duty to lead a blameless life in the sight of God: ‘May the Lord be generous in increasing your love and make you love one another and the whole human race as much as we love you. And may he so confirm your hearts in holiness that you may be blameless in the sight of our God and Father when our Lord Jesus Christ comes with all his saints’ (1 Th 3:12-13).” — from his address to a symposium on “Holiness in Christianity and in Islam,” Rome, May 9, 1985