The medieval immanentism that Voegelin was talking about must have been Joachimism, or at least that is what de Lubac would have said. For de Lubac, the Gnosticism of contemporary attempts to immanentize the eschaton (in such things as certain strands of political theology, for instance, some passages in the writings of Schillebeeckx which de Lubac criticizes explicitly in A Brief Catechesis on Nature and Grace) is a continuation of Joachimism, and therefore is a medieval heresy. See especially La Postérité spirituelle de Joachim de Flore.
Also, doesn’t this passage describe the problem with Wahabbism?
When people think they possess the secret of a perfect social organization which makes evil impossible, they also think that they can use any means, including violence and deceit, in order to bring that organization into being. Politics then becomes a "secular religion" which operates under the illusion of creating paradise in this world. But no political society — which possesses its own autonomy and laws — can ever be confused with the Kingdom of God. The Gospel parable of the weeds among the wheat (cf. Mt 13:24-30; 36-43) teaches that it is for God alone to separate the subjects of the Kingdom from the subjects of the Evil One, and that this judgment will take place at the end of time. By presuming to anticipate judgment here and now, man puts himself in the place of God and sets himself against the patience of God
On another note, if Voegelin did stray from Christian orthodoxy in later years I think it would be important to show how that straying at least was a partial rejection of this insight, lest someone (and there are people out there who are just itching for the chance) would then conclude: “Ha! See? De Lubac was a heretic!”