Q: What is faith in Jesus Christ?
A: Faith in Jesus Christ is a saving grace, whereby we receive and rest upon him alone for salvation, as he is offered to us in the gospel.

Hebrews 10:39. But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul.
John 1:12. But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name.
Isaiah 26:3-4. Thou wilt keep him in perfect peace, whose mind is stayed on thee; because he trusteth in thee. Trust ye in the Lord for ever: for in the Lord Jehovah is everlasting strength.
Philippians 3:9. And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith.
John 6:40. And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.
Galatians 2:16. Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the works of the law: for by the works of the law shall no flesh be justified.

Westminster Project - On the Properties, Means, and Signs of Faith by John Flavel