For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, Ephesians 2:8 NKJV
Therefore it is of faith that it might be by Grace; Romans 4:16a KJV
Faith is being fully persuaded by unseen evidence. That evidence is what the Finished Work procured for us, and what God said to us because of the Finished Work. Faith cannot exist where the will of God is not known. That's why God in His infinite Goodness has let us know what His will is.