I found the original text and this is what the speaker, a murderer, says a few sentences before:
'The outside world means nothing, only the interior world is real. It is utterly compelling, compulsive. Nothing can pull you back from the edge. Not laws or threats of punishment, morality, religion, fear of death. All of those things are as meaningless as the life that you're about to extinguish.'
This seems to confirm the idea that this is about not being challenged in the sense of something requiring your best abilities and being a potential respectable achievement, but in the sense of having the challenge to you from those who object (or have the potential to object) to your actions. In my dictionary it gives a good and concise definition of the verb 'challenge' in this sense as 'to invite to prove or justify something' and of the noun 'challenge' as 'a summons to prove or justify something'. That nuance, 'summons' is good in pushing it beyond 'invite' to something more authoritative, cooercive or just dictated by circumstances.
He has crossed that line of murdering someone and now feels unrestrained by any appeal to him from any law, morality, religion, fear, whatever, i.e. 'unchallenged by divine or secular power'. So something with the idea of 'unlimited' or 'unrestrained' might be reasonable, but loses some hue of meaning. Creating a phrase to convey the idea seems necessary.
Maybe something like:
Но тогда найдёшь, что больше не сдерживает тебя божественная или светская сила.
If you're really just looking for the other meaning of 'challenging' or 'unchallenging' as I mentioned above, then in all my time in Russia I've never encountered a complete translation of it without some kind of explanation seeming necessary to be absolutely precise. Like 'unchallenging work' would just perhaps be 'нетрудная и неинтересная работа' maybe too you could say that such work 'не соответствует моим способностям', i.e. 'is not in accordance with my capabilities'.
But it's clear to me, as a native English speaker with a lot of experience in making precise sense of texts, that in this text it really isn't that meaning, but the one of no longer being asked to justify himself, no longer being called to account by, being held back by, any divine or secular power.
Maybe in fact those concepts could be used, i.e. to no longer have to answer for or account for yourself before any divine or secular power; like:
Но тогда найдёшь, что уже не приходиться отчитываться перед божественной или светской силой.