Assurance

Last night I talked to a young lady about her salvation. She believed that after every sin, she would have to re-accept Christ as her Savior. Does anyone have any verses or ideas you can share, so I can better explain things to her?
We talked about God holding us in His hand & never letting go. Then I explained that even if she did something wrong against her mom, she would still be her mother's daughter. I'm just wondering if there's a passage I could take her to so she could go there when she starts wondering. She regularly attends a church that preaches you can lose your salvation.
The only verses I found dealing with "assurance" were in 1 John that talk about your testimony proving that Christ is working in you.
Please remember this is a 13 year old girl, so I don't want to hand her a seminary type textbook, but maybe just a note to let her know I care.
Thanks!