'This is the work of God, that you believe in the one whom he has sent.' --John 6:29
Bavinck on faith:
[I]t occupies such a central place that it can be called 'the work of God' par excellence (John 6:29). It is the principle of the Christian life as a whole, the means by which we obtain Christ and all his benefits, the subjective source of all salvation and blessing. While through Scripture it binds us to the historical Christ, it at the same time lifts us up to the invisible world and causes us to live in communion with the Lord from heaven. Wherever faith may be rooted in human beings, it affects all our capacities and powers, gives us direction and guidance, controls our intellect and heart, our thinking and activity, our life and conduct. Christians are believers. Faith is mystical and noetic, receptive and spontaneous, passive and active, the opposite of all works and itself the work of God par excellence, the means of justification and the principle of sanctification, accompanying us throughout our lives and changing into sight only at death.
--Herman Bavinck, Reformed Dogmatics, 4:121-22