I recently read a great post on the subject of deliberate sin by Dan Delzell a Lutheran Minister from Nebraska.
Here is the link to the article: http://www.christianpost.com/news/can-deliberate-sin-negate-your-conversion-experience-76301/
So is someone LOST who deliberately sins?
The truth is we as human's can't tell by looking at someone if the change has occurred in their life....has Christ indeed changed - regenerated the person?
The daily walk of the person with Christ is the best indicator of change...being genuinely Born Again.
Dan Delzell points out the trusting and obeying are the only ways to walk in Christ.
Rather than pointing to a particular time we prayed a "special prayer" as a the proof of being a Christian...the pattern of good fruit and continued faith in our life is the best indicator of how rooted we are in Christ.
Ken