One of the Fruits of the Holy Spirit. A visible attribute of a true Christian life as found in Galatians 5:22-23.
Faithfulness is committing oneself to something or someone. Being faithful requires personal resolve not to wander away from commitments or promises.
"I pray that out of his glorious riches he may strengthen you with power through his Spirit in your inner being, so that Christ may dwell in your hearts through faith". Ephesians 3:16-17