Faith is confidence in what is held to be true. For this reason, it is based on verifiable testimony from trustworthy sources. The history of the people of God has many examples of those who trusted God, the source of all truth. The writer to the Hebrews provides a chronological list of these "heroes of the faith".
Faith is the ability to hold tight to that which is trusted even when definitive proof has not been presented yet. As a spiritual gift, it is not available to those who reject God, but once made available by God it opens one to come to God and be saved.
Ordinarily, the message of the gospel must be heard in order for someone to believe it. After God graciously awakens the faith within a person, they will trust His Word and then show that they believe Him by their good works.
There are degrees of faith (trust) which people can demonstrate. On one level, there is an intellectual assent to what is known to be true. For, instance, knowing that there is one true God is a good thing, but trusting Him enough to change is beyond the ability of those not touched by the grace of God. On a spiritual level, faith that changes one's nature results in good works such as those of Abraham and Rahab.
The collected beliefs of believers is known as "the Faith" which defines Christians. This faith is accepted as absolute truth, for which persecution and even death is to be expected. These beliefs, to be binding to the conscience, must be drawn from the Bible.