What is the mark of a Christian? To be born anew through baptism of water and the Spirit. What is the mark of one born of water? That he be dead and immovable with regard to all sin, as Christ died once and for all because of sin, as it is written: ‘all we who are baptized in Christ Jesus are baptized in his death. For we are buried together with him by baptism unto death; knowing this, that our old man is crucified with him that the body of sin may be destroyed, to the end that we may serve sin no longer.’ [Rom. 6,3,4,6] What is the mark of one born of the Spirit? That he become in the measure granted him that of which he has been born, as it is written: ‘That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.’ [John 3,6] What is the mark of him who has been born anew? That he strip off the old man with his deeds and cupidities and put on the new man, ‘who is renewed unto knowledge, according to the image of him that created him.'[Col. 3,10] as it is written: ‘As many of you as have been baptized in Christ have put on Christ.'[Gal. 3,27] What is the mark of a Christian? That he be purified of all defilement of the flesh and of the spirit in the Blood of Christ, perfecting sanctification in the fear of God and the love of Christ, [2 Cor. 7,1] and that he have no blemish nor spot nor any such thing; that he be holy and blameless [Eph. 5,27] and so eat the Body of Christ and drink His Blood; for ‘he that eateth and drinketh unworthily, eateth and drinketh judgment to himself.’ [1 Cor. 11,29] What is the mark of those who eat the Bread and drink the Cup of Christ? That they keep in perpetual remembrance Him who died for us and rose again. What is the mark of those who keep such remembrance? That they live not for themselves but for Him who died for them and rose again. [2 Cor. 5,15] What is the mark of a Christian? That his justice abound in all things more than that of the scribes and Pharisees, according to the rule of the doctrine which has been handed down in the Lord’s Gospel. [Matt. 5,20] What is the mark of the Christian? That they love one another as Christ has loved us. [Eph. 5,2] What is the mark of a Christian? To set the Lord always in his sight. [Ps.15,8] What is the mark of a Christian? To watch daily and hourly and stand prepared in that state of perfection which is pleasing to God, knowing that at what hour he things not, the Lord will come. [Luke 12,40]
Saint Basil the Great, “The Morals,” 80, 22 in Saint Basil. Ascetical Works. Translated by Sister M. Monica Wagner, C.S.C. (New York: Fathers of the Church, Inc., 1950), 204-205.