"If you're not a Christian anymore, you never truly were"- isn't that a bit of a cop-out?

Say that someone dedicates their life to the lord, is saved and believes, and lives as a Christian. But they ask questions and evaluate him/herself and decides they don't believe. Wouldn't that person have the authority to say, "I did try your lifestyle and it doesn't suit me?" Why be dismissive and say, "you must've done it wrong?" Doesn't that make Christianity seem more cult-like, to say, "Anyone that leaves can't claim they ever believed. Anything they say on Christianity is invalid." Although that person experienced it firsthand.