It is equally faulty to trust everyone and to trust no one.
[Utrumque enim vitium est et omnibus credere et nulli.]
Seneca the Younger (c. 4 BC-AD 65) Roman statesman, philosopher, playwright [Lucius Annaeus Seneca]
Moral Letters to Lucilius [Epistulae morales ad Lucilium], letter 3 “On True and False Friendship” [tr. Gummere]
(Source)
Alt. trans.: "For it is both a vice to believe everyone and no-one."