And God, who knows the heart, bore witness to them, by giving them the Holy Spirit just as he did to us, and he made no distinction between us and them, having cleansed their hearts by faith.
Faith is a response of the heart which receives what God has already done for us in Christ. Faith is relying on God’s character, standing on God’s promises, believing God’s Cross, and obeying God’s Spirit with a certainty that surpasses physical sight and human reasoning. Faith ignores bad circumstances, negative feelings, or discouraging thoughts to stand on God’s word and walk in his ways (Isa. 55:8-9). In short, faith simply believes what God says is true.
True faith passively receives the benefits of Christ’s victory on the cross resulting in active obedience to Christ’s commands and acquiescence to the leading of the Holy Spirit. Saving faith does not involve meriting salvation by human work. However, genuine faith will bear good fruit: an expression of the life of Christ in us. Good deeds are not the foundation of our acceptance with God, but the correct response and fruit of a living relationship with him.
Faith is God’s work in us, that changes us and gives new birth from God. (John 1:13). It kills the Old Adam and makes us completely different people. It changes our hearts, our spirits, our thoughts and all our powers. It brings the Holy Spirit with it. Yes, it is a living, creative, active and powerful thing, this faith.
Faith cannot help doing good works constantly. It doesn’t stop to ask if good works ought to be done, but before anyone asks, it already has done them and continues to do them without ceasing. Anyone who does not do good works in this manner is an unbeliever. He stumbles around and looks for faith and good works, even though he does not know what faith or good works are. Yet he gossips and chatters about faith and good works with many words.
Martin Luther, Martin Luther’s Definition of Faith: An Excerpt, “An Introduction to St. Paul’s Letter to the Romans,” Luther’s German Bible of 1522.