The teachings of Christ spoke of confessing your sins. Of how God has mercy on those who ask for forgiveness. Early Christians distinguished between lesser, daily sins and greater, death-dealing sins. Lesser sins could be forgiven through prayer, fasting, works of mercy, and the Eucharistic celebration. Greater sins required a process of reconciliation.
Tertullian, in De Poenitentia, called it the “second penitence” (baptism being the first) and said it could be received only once.