Imagine this scene for a moment. A woman notices her husband forgot to shut off his computer. She goes over to it with the intention of turning it off. To her shock and dismay she discovers that the computer is logged on to a pornographic website. She quickly does a search of sites visited and she’s horrified. He promised her that he had stopped. But then again he’d broken that promise on more than one occasion in the past. She immediately confronts him. His face is guilt ridden---his brow sweaty, his voice shaky. She shouts in anger and hurt, "Why ---why should I keep on believing you?"
Switch now to another imaginary scene. A young girl comes in an hour late from a date. She claims she would have phoned home but the car they were in stalled and they weren’t anywhere near a phone and what’s more they were apparently in a pocket where her cell phone wouldn’t work. This young girl has always tried to be upfront and honest with her parents. Her parents can see that her boyfriend’s hands are covered with engine grease and they’d already overheard him on the phone asking his Dad if he could come pick him up and help him work on his car. "Well, of course we believe you. Why wouldn’t we believe you, after all you’ve not given us reason to doubt you." the girl’s parents tell their daughter.
When you put those two illustrations alongside each other you realize that the amount of one’s faith or trust in another person is usually related to a track record. Since that husband had a poor track record it would be reasonable for his wife to have doubts. On the other hand the young girl had a proven track record so it was reasonable for her parents to believe in her. God’s proven track record is an integral part of the faith equation.