The truth

When Ren heard Valdel asked him the same question, the implications of the question had changed. If it was before Valdel simply sounded like he had a hunch, but now after explaining all that it was obvious that he pretty much knows the whole truth. 

If he simply said that he thinks Ren was a descendant of Demon Lord Kretos, then Ren would've gone with that. Yet now he actually knew about the term the strongest mortal soul. That was something that not even some deities know of.

Ren looked into Valdel's eyes, it would seem that only a little bit of malice remained. Ren's answer would now determine what would happen to Valdel. If he lied at this point Valdel would hide his emotions deep inside and might become the same as the old Shadow Clan that Ren was a part of when he was still Kretos. 

Ren was certain that if he lied at this point Valdel would change and be consumed by the Primordial Demon's malice.