In honor of The Screwtape Letters, with apologies to C.S. Lewis:
To begin, we must first outline some of the differences between old converts and new believers. You would think new believers would be the easiest to tempt, but this is not always the case. Often times old Christians are much easier than new ones.
New believers may be unaccustomed to the language and behaviors of Christians, but that is of little use to us. They may have some old bad habits – and you can certainly exploit those, but what they do have is a recent fire in the belly – a fresh realization of our Rival’s holiness and their own sin. Many older believers have gone years without those thoughts. Even better, their religious habits have produced a self-satisfied version of devoutness you will find cocoons them completely.
As for the new convert, try attacking those old habits and while it may succeed in the moment, the counter-effect is that it reminds them of their sinfulness and their recent decision and drives them wholly to our Rival’s camp. You may win the battle, but lose the war, and we are taking the long-term view here.
What you can try to do, is make the young disciple question their previous conversion. Was it real, or some imaginary excitability of the moment? Have the initial feelings lingered? Is he really “changed?” Where is the proof of this? Feel free to employ feelings, legends and fairy tales – anything to make the conversion seem fanciful and completely unscientific.