Here is a sound definition from James I. Packer in his classic book Knowing God in the chapter “Thy Word is Truth:”
“A person who acknowledges and lives under the Word of God. He submits without reserve to the Word of God written in ‘the Scripture of truth’ (Daniel 10:21), believing the teaching, trusting the promises, following the commands. His eyes are to the God of the Bible as his Father, and the Christ of the Bible as his Savior. The Word of God has convinced him of sin and assured him of forgiveness. He aspires to have his whole life brought into line with it. The promises are before him as he prays, and the precepts are before him as he moves among others.
He knows that in addition to the Word of God spoken directly to him in the Scriptures, God’s Word has also gone forth to create, and control, and order things around him; but since the Scriptures tell him that all things work together for his good, the thought of God ordering his circumstances brings him only joy. He is an independent fellow, for he uses the Word of God as a touchstone by which to test the various views that are put to him, and he will not touch anything which he is not sure that Scripture sanctions.”
J.I. Packer asked back in 1973: “Why does this description fit so few of us who profess to be Christians these days?” He concludes: “You will find it profitable to ask your conscience, and let it tell you.”