To have faith is to believe in something or someone. We use this word in a way that’s similar to the noun, "hope." It’s also often used to describe a religious institution and the people who practice a religion.
Yussuf has a strong faith in God.
His faith in God is strong.
Sabeen’s faith was tested during the war.
The United States is home to people of many different faiths.
Meredith and Chris have a strong marriage and they have faith in each other.
I have faith that you will be successful.
My faith in you is strong.
Tom put too much faith in the knot that he tied for his boat, and it broke loose.
Jennifer has no faith that her car will start.
Bill has very little faith in the weather forecast for today.
Adding "ful" to the end of "faith," makes the adjective, "faithful."
They have been faithful to each other.
Benito is a faithful and devoted husband.
Susan was unfaithful to her husband, so he divorced her. (To make the opposite of "faithful," add a "un" prefix.)
Note: Sometimes the word "faithful" is used as a noun: Every Sunday the faithful gather at this church to worship God.