Donatism: During the fourth and fifth centuries AD, a movement arising out of the initial teaching of Donatus sought to separate the “pure” church from the “apostate,” or “fallen,” church. The Donatists vigorously opposed Christian involvement in the military because the military was seen an instrument of the evil state. Donatists also urged that Christians be rebaptized if they had been baptized by a bishop who had cooperated with the pagan emperor. *Augustine eventually refuted the Donatist arguments.
Stanley J. Grenz;David Guretzki;Cherith Fee Nordling. Pocket Dictionary of Theological Terms (Kindle Locations 427-430). Kindle Edition.
Expiation. The belief that sin is canceled out by being covered over. For Christians, expiation suggests that Christ’s death covers our sins. Biblical scholars debate whether the Greek terms deriving from hilaskomai should be translated as “*propitiation,” denoting the turning away of divine wrath, or, in contrast, as “expiation,” denoting the sense of covering sins or canceling a debt. Hilaskomai is also sometimes translated simply as “mercy seat.” 
 Stanley J. Grenz; David Guretzki; Cherith Fee Nordling. Pocket Dictionary of Theological Terms (Kindle Locations 533-535). Kindle Edition.
Communion: Generally, a term most closely related to the biblical idea of fellowship (from Greek *koinania). It can refer to either the relational fellowship persons may have with God or the fellowship persons have with one another, especially with those who are in Christ. The term is also used in reference to the *Lord’s Supper as an event marking the fellowship of the participants with Christ and with each other.______________________