What is a Christian? Christians can be described from many angles, but… it is clear that we can cover everything by saying: True Christians are people who acknowledge and live under the word of God. They submit without reserve to the word of God written in “the Book of Truth” (Dan 10:21), believing the teaching, trusting the promises, following the commands. Their eyes are upon the God of the Bible as their Father and the Christ of the Bible as their Savior.
Christians will tell you, if you ask them, that the Word of God has both convinced them of sin and assured them of forgiveness. Their consciences, like Luther’s, are captive to the Word of God, and they aspire like the psalmist, to have their whole lives brought into line with it. “Oh, that my ways were steadfast in obeying your decrees!” “Do not let me stray from your commands.” “Teach me your decrees. Let me understand the teaching of your precepts.” “Turn my heart toward your statutes.” “May my heart be blameless toward your decrees” (Ps 119:5, 10, 26-27, 36, 80). The promises are before them as they pray, and the precepts are before them as they go about their daily tasks.
Christians know that in addition to the word of God spoken directly to them in the Scriptures, God’s word as also gone forth to create, and control, and order things around them; but since the Scriptures tell them that all things work together for their good, the thought of God’s ordering their circumstances brings them only joy. Christians are independent folks, for they use the Word of God as a touchstone by which to treat the various views that are put to them, and they will not touch anything which they are not sure that Scripture sanctions.
Why does this description fit so few of us who profess to be Christians in these days? You will find it profitable to ask your conscience, and let it tell you.
J.I. Packer, Knowing God, p. 116