What is Faith?
True faith is not only a certain knowledge whereby I hold for truth all that God has revealed to us in His Word; but also a hearty trust, which the Holy Spirit works in me by the Gospel, that not only to others, but to me also, forgiveness of sins, everlasting righteousness and salvation are freely given by God, merely of grace, only for the sake of Christ's merits.
In fact, we find that there are two elements to true faith: (i) intellectual assent to the truth, and (ii) trust.
This means knowing the facts of the gospel and believing that they are true. In order to have faith in Christ and His redemptive work we need to know the facts about who he is and what he has done (i.e. the gospel). Not only do we need to know these facts, but we also need to accept that they are true. Yet true biblical faith is more than mental assent to the facts of the gospel. Thus there is another essential element to true saving faith.
This is the realisation that Jesus didn’t just die for people in general, or for other people, but for me. This is when, in the realisation that my soul is guilty and defiled by sin, I look to Christ for the forgiveness which comes only through His atoning work. In other words, this is trusting Christ for salvation, what the Heidelberg Catechism refers to as ‘a hearty trust … that … forgiveness of sins, everlasting righteousness and salvation are freely given by God, merely of grace, only for the sake of Christ's merits.’
Acts 16:31 provides us with the response: ‘Believe on the Lord Jesus Christ, and you will be saved’. The Lord Jesus Christ, God the Son who took on human flesh and died for our sin, rising again on the third day, is the object of our faith. We believe in Him ‘who became for us wisdom from God—and righteousness and sanctification and redemption’ (1 Corinthians 1:30).
 Heidelberg Catechism 21 (Lord’s Day 7)