Topics:

• Eucharist: specifically the Real Presence and the function of the Eucharist

• Mary: praying to her, her sinlessness, etc.

• Saints: praying to them, adopting patron saints

• Christus Victor: as opposed to the later development of looking at sin and Jesus' death/resurrection as "payment" for our "debt"

• Other: icons, justification, sanctification

Clarifications:

• I'd prefer Ante-Nicene Fathers (so up to 325 CE), but anything is acceptable

• Also please list specific writings of the Fathers, rather than just their name

• Feel free to explain WHY the Father/writing was influential