Abraham’s purpose needed the quality of continuity and perpetuity, for time is tripartite, taking in, as it does, the past, the present, and the future.
By this we are admonished that there should not be any trace of forgetfulness of the beneficence of God whether in the past, present, or future.
We should, rather, be steadfast in the recollection of His grace and in our compliance with His command.
Another reason for this reference to time lies in the fact that the person who performs a sacrifice ought to put his trust in the brilliant light of the Trinity.
For him whose sacrifice is grounded in faith has ever around him the light of day. For him there is no night.
So in Exodus Moses says: ‘We will go three days’ journey to sacrifice unto the Lord our God’ (2 Exod. 3:18).
Elsewhere, too, when God appeared to Abraham by the oak of Mamre, we are told that ‘Abraham raised his eyes and saw three men standing at a distance from him.
As soon as he saw them he ran to the entrance of the tent door to meet them and bowed down to the earth and said: My Lord, if I have found favor with you’ (Gen. 18:2, 3).
He beholds three and one he adores. He offers three measures of fine flour (cf. Gen. 18:6).
Although God is immeasurable, He nevertheless holds the measure of all things, as it is written: ‘Who hath measured the waters in his hand and weighed the heavens with his palm and the bulk of the earth in the hollow of his hand? (Isa. 40:12).
The holy patriarch, therefore, offered sacrifice in the secret recesses of his heart to the Trinity made perfect in each of the Persons.
This is the spiritual meaning of the measures of fine flour. This is the measure of fine flour mentioned in the Gospel which was ground by the woman who ‘will be taken’. ‘One will be taken; the other will be left’ (Matt. 24:41).
The Church ‘will be taken’; the Synagogue ‘will be left’, or the man of good conscience will be taken and the man of bad conscience, left.
That you may know that Abraham believed in Christ, we read; ‘Abraham saw my day and was glad’ (John 8:56).
He who believes in Christ believes, too, in the Father, and who believes in the Father believes, too, in the Son and Holy Spirit.
There were three measures, therefore, and one substance of fine flour. This means that there was one sacrifice which was offered to the Blessed Trinity.
Ambrose of Milan (c. 337-397): Cain and Abel, book 1, chapter 8, 29-30, in St Ambrose: Hexameron, Paradise, and Cain and Abel, tr. John J. Savage, Catholic Univeristy of America Press, 1961, pp. 386-388.