Faith is just believing,

What God says He will do;
He will never fail us;
His promises are true.
If we but receive Him,
His children we become;
Faith is just believing this
Wondrous thing is done.

Mrs. W. B. Liggett

(children's song)

And brought them out, and said, Sirs, what must I do to be saved?

And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house. 

(Acts 16:30-31)

For therein is the righteousness of God revealed from faith to faith: as it is written,

The just shall live by faith. 

(Romans 1:17)