'Sin' is used to mean a sacrifice for sin, see 10039. 'Expiation' means purification from evils and consequent falsities, 9506.

The reason why this purification is accomplished by means of truths which spring from the good of innocence is that the blood with which the expiation was accomplished came from a young bull or from a lamb, and 'a young bull' means the good of innocence in the external man, 9391, 9990, 10132, 'a lamb' the good of innocence in the internal man, 10132; and innocence must be present if truth and good are to be received, 3111, 3994, 4797, 6013, 6765, 7836(end), 7840, 9262, 10134, as well as the places referred to in 10021. The good of innocence consists in acknowledging that all truths and forms of good come from the Lord and none at all from the human self or proprium; thus it consists in wishing to be led by the Lord and not by self. From this it is evident that the more a person trusts and believes in himself, thus the more he is ruled by self-love, the less the good of innocence is present in him. This is why a person cannot be purified from evils unless the good of innocence is present in him. For if this good is not present that person is led not by the Lord but by self; and anyone who is led by self is led by hell, since the human proprium is nothing but evil, and all evil belongs to hell. The fact that every expiation was accomplished with the blood either of a young bull, or of a lamb, or of turtle doves, that is, young pigeons, is clear in Moses, in Exod 29:36; Lev 4:1-7,13-18,27-end; 5:1-7; 15:14,24,28-31; Num 6:9-11. 'Turtle doves' and 'young pigeons' as well mean the good of innocence.