Sin destroys the relationship between God and people, and atonement is the means by which a reconciliation is effected. In the OT, sacrifices and offerings are prescribed and were well developed in the post-exilic period with a view to removing the barrier caused by sin which cut a person off from God's favour. Sacrifices were appointed by a merciful God for the restoration of fellowship which an individual by his own personal act was incapable of achieving. During the Persian period (551–323 BCE) it would seem that 1,000 lambs and 100 bullocks would have been required for the stipulated ceremonies; but it is doubtful whether, in fact, the Judean highlands could have supplied such a large number. In the time of the Maccabees animal sacrifices were thought to be supplemented by the atoning value of human suffering. In the stories of Maccabean martyrs their sufferings are held to be an atonement not only for the sufferers but also for others. So the youngest of the seven brothers appeals to God that through him and his brothers the wrath of the Almighty which has fallen on the nation may be brought to an end (2 Macc. 7: 38). On the Day of Atonement (Yom Kippur = day of covering, Lev. 16) the offering of a bull permitted Aaron to approach the mercy-seat in the holy place; and a goat ‘for God’ was sacrificed to atone for the pollution of the holy place, by reason of the sins of the people. Aaron laid his hands on a second goat ‘for Azazel’, confessing the sins of Israel, and this goat was then dispatched into the wilderness bearing away all the sins of Israel (Lev. 16: 21), cleansed from defilement. Broken relationships between individuals were to be repaired by T'shuvah (returning; repentance).
In the NT atonement is connected with the life, death, and resurrection of Jesus and he may have understood his coming death in atonement terms (Mark 10: 45). In Paul's letter to the Romans the source of God's mercy in reconciling mankind was Jesus Christ (Rom. 3: 25) and in the letter to the Hebrews the notion of sacrifice is employed; the defilement of sin is cleansed by the blood of a sinless victim. The blood of Christ's obedient and offered life is sprinkled on our sinful consciences so that we can then draw near to God with a similar obedience, for this sacrifice did not suffer from the imperfections rising from the ordinances of the old covenant. The former sacrificial victims were only unstained by sin because they were animals (Heb. 10: 4) and were sacrificed against their will. Christ's sacrifice was freely given, in total obedience to the Father, and it serves to change the attitudes of humankind to God (Heb. 12: 2), that is, to remove the barrier which sin creates (Heb. 10: 22).
We are changed; not God. The theory of ‘penal substitution’—that human beings are let off the consequences of sin by reason of Jesus' crucifixion having appeased the Father on our behalf—is incompatible with the doctrine of the Trinity which articulates the initiative of God's coming among us in Jesus.