Romans 10:9 is a good answer:
If you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved.
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.
The web site gotquestions.org offers another good response:
The Bible teaches that the good works we do cannot make us acceptable to God. Titus 3:5 says, “He saved us, not because of righteous things we had done, but because of his mercy. He saved us through the washing of rebirth and renewal by the Holy Spirit.” So, a Christian is someone who has been born again by God (John 3:3; John 3:7; 1 Peter 1:23) and has put faith and trust in Jesus Christ. Ephesians 2:8 tells us that it is “…by grace you have been saved, through faith—and this not from yourselves, it is the gift of God.”
A true Christian is a person who has put faith and trust in the person and work of Jesus Christ, including His death on the cross as payment for sins and His resurrection on the third day. John 1:12 tells us, “Yet to all who received him, to those who believed in his name, he gave the right to become children of God.” The mark of a true Christian is love for others and obedience to God’s Word (1 John 2:4, 10). A true Christian is indeed a child of God, a part of God’s true family, and one who has been given new life in Jesus Christ.