The current English translation, “There is every reason to doubt that it is true”, is dubious. The meaning is rather “There is every reason to suspect that it is true”.
This is amazing. Epäillä is a verb that can have two opposite meanings. Who came up with that? Lol.
Anyway, wiktionary has some usage hints, and after reading those, I would also agree that “suspect” is more likely here:
The meaning “to suspect” is more common with subordinate clauses using [että] that have the verb in indicative mood, and is practically the only interpretation when the genitive of a participle is used (generally considered fairly formal).
Epäilen, että teit sen. ― I suspect you did it.
Epäilen sinun tehneen sen. ― I suspect you did it.
The meaning “to doubt” is more common with the verb in conditional mood, and is practically the only interpretation with tag questions formed with [-ko].
Epäilen, että olisit tehnyt sen. ― I doubt you would have done it. (but also possibly I suspect you would have done it.)
Epäilen, teitkö sitä. ― I doubt (whether) you did it.