About the doctrine of the Trinity:  this finds abundant attestation in 
the most ancient Church Fathers.  A superb resource is Jurgens' Faith of 
the Early Fathers (three vols., about $45 from Eighth Day Books), which 
contains 1046 theological assertions with citations to passages from the 
Fathers, excerpted in the three volumes, that make this assertion.  Thus 
for the doctrine "That God is triune is clear in the Tradition of the 
Church even  in the pre-Nicene era," Jurgens offers no fewer than 39 
passages, drawn from Clement of Rome, Ignatius of Antioch, "The Martyrdom 
of Polycarp," Dionysius Bishop of Corinth, Aristides of Athens, Justin 
Martyr, Athenagoras of Athens, Theophiilus of Antioch, that wonderful 
writer Irenaeus of Lyons, Tertullian, Hippolytus of Rome, Clement of 
Alexandira, Origen, Cornelius of Rome, Cyprian of Carthage, "Pope" 
Dionysius, Gregory Thaumaturgus, Aphraates the Persian Sage, and 
Athanasius.

Incidentally, one fascinating thing about this set is how the chief 
orthodox doctrines of God, etc., find abundant early attestation, while 
matters such as the bodily assumption of the Lord's mother into heaven is 
first attested in Gregory of Tours (about AD 590).