Psalm 78 is a wisdom Psalm about Ephraim turning into his brother, Manasseh.
Ephraim’s failure to come to Jephthah’s aid (Judges 12:1-6) initiates the Psalm’s review of Israel’s stubbornness and rebellion (Psalm 78:8-10). Ephraim refused to take up arms (v. 9), a sign that Ephraim had forgotten the Exodus (v. 11). The exodus should have inspired Ephraim to fight. After a long series of rebellions, Yahweh rejects Ephraim (v. 67) and instead chooses a king from the tribe of Judah (vv. 68-71). Within the Psalm, the review of the events of the exodus and wilderness wandering is framed by references to Ephraim’s forgetfulness, Ephraim’s tendency to become Manasseh, whose name means forgetfulness (cf. Genesis 41:51).
The Psalm also assumes that Ephraim should have remembered and taken courage from events that had happened hundreds of years before. In short, the Psalm assumes a generational continuity. The condemnation of Ephraim makes no sense unless Ephraim is the same people that had been saved long before in the exodus. The wisdom of this maskil is that what God did in the past for ancestors, He can and will do again for His people who trust in Him.
The church has the same sort of inter-generational continuity: Promises given to past generations apply to present generations. Victories won in the fourth century should inspire faithfulness in the 21st, because the church is one and catholic in time as in space.
And the sign of this inter-generationality is baptism.