To piggyback off that, "achar" and "crer" are different but mirror the verbs "think" and "believe" in English. To wit, these sentences mean the same thing in English or Portuguese:
- Acho que sim = I think so
- Creio que sim = I believe so
However, you should use "crer" when speaking of more fixed or fundamental beliefs. For example, "acho em Deus" makes no sense; it's like saying "I think in God" in English. Rather, you'd say, "creio em Deus," meaning "I believe in God."
To make it just a little bit more complicated, "achar" can also mean "to find (something)." "Encontrei a chave perdida" and "achei a chave perdida" are synonymous.