Yes. How does one explain the teaching of the Shammaites:
Be aware that the Shammaites were just one of the Judaistic sects of the 1st century who fought and warred with others over minute and obscure points of Jewish doctrine:
Be aware that any teaching that the souls of people in the Old Testamental period were in purgatory is heresy in Roman Catholic teaching. The Catholic Church teaches that they were in the Limbo Patrum.
So, the question remains: WHEN did God create purgatory?
I thought that the Pope had mentioned Purgatory in relation to the Old Testament religious law. for example, at the General Audience of Wednesday, 4 August 1999, we read JPII:
According to Old Testament religious law, what is destined for God must be perfect. As a result, physical integrity is also specifically required for the realities which come into contact with God at the sacrificial level such as, for example, sacrificial animals (cf. Lv 22: 22) or at the institutional level, as in the case of priests or ministers of worship (cf. Lv 21: 17-23). Total dedication to the God of the Covenant, along the lines of the great teachings found in Deuteronomy (cf. 6: 5), and which must correspond to this physical integrity, is required of individuals and society as a whole (cf. 1 Kgs 8: 61). It is a matter of loving God with all one's being, with purity of heart and the witness of deeds (cf. ibid., 10: 12f.)
The need for integrity obviously becomes necessary after death, for entering into perfect and complete communion with God. Those who do not possess this integrity must undergo purification. This is suggested by a text of St Paul. The Apostle speaks of the value of each person's work which will be revealed on the day of judgement and says: "If the work which any man has built on the foundation [which is Christ] survives, he will receive a reward. If any man's work is burned up, he will suffer loss, though he himself will be saved, but only as through fire" (1 Cor 3: 14-15).
3. At times, to reach a state of perfect integrity a person's intercession or mediation is needed. For example, Moses obtains pardon for the people with a prayer in which he recalls the saving work done by God in the past, and prays for God's fidelity to the oath made to his ancestors (cf. Ex 32: 30, 11-13). The figure of the Servant of the Lord, outlined in the Book of Isaiah, is also portrayed by his role of intercession and expiation for many; at the end of his suffering he "will see the light" and "will justify many", bearing their iniquities (cf. Is 52: 13-53, 12, especially vv. 53: 11).
Psalm 51 can be considered, according to the perspective of the Old Testament, as a synthesis of the process of reintegration: the sinner confesses and recognizes his guilt (v. 3), asking insistently to be purified or "cleansed" (vv. 2, 9, 10, 17) so as to proclaim the divine praise (v. 15).