What is Faith?
The object of Christian faith is invisible and not susceptible to observation. If a thing can be immediately observed by us, faith is superfluous; faith is opposed to sight (Rom. 8:24; 2 Cor. 5:7). This does not conflict with the fact that revelation certainly took place in space and time and that the person of Christ could be seen and touched. For as the object of faith this revelation as a whole was not observable. Many people saw Jesus and still did not believe in him; only his disciples saw in him “a glory as of the only begotten of the Father” (John 1:14). Word and deeds are the object of faith only when considered from a divine perspective. But in Scripture, [pistis], as saving faith, acquires an even more pregnant meaning; its object is not all sorts of words and deeds of God as such but the grace of God in Christ (Mark 1:15; John 3:16; 17:3; Rom. 3:22; Gal. 2:20; 3:26; etc.). To [Christian] faith this special object is considered under still another heading than that of truth versus falsehood. The universal nature of faith is not exhausted by being characterized as a firm and sure knowledge, an objective holding for truth, since it also includes a heartfelt trust in a total surrender to God, who has revealed himself in Christ, and a personal appropriation of the promises extended in the gospel.
-Herman Bavinck, Reformed Dogmatics, Vol. 1, 568.
For another perspective, see Greg Koukl’s answer.