As I get older it seems that I believe fewer and fewer things are true. However, the things that I do believe are true, I believe more and more that they are true.
As I get older it seems that I believe in fewer and fewer things. However, the things that I do believe in, I believe in more and more.
Does that help? As you can see, there are two possibilities with "believe". Your original gave "believe" but I have given the "believe in" version too. I think the latter fits better with the sentiment of the piece but I don't know what the original author was thinking.
Talking of the original author, please give the source of your quote.
Student or Learner