How does he demonstrate it?
By Cicero's argument,--that is, by the consent of the human race.
There is nothing new in that.

We have still to find out whether the belief of the human race is legitimate; or, as Kant says, whether our subjective certainty of the existence of God corresponds with the objective truth.

This, however, does not trouble M.Lamennais.He says that, if the human race believes, it is because it has a reason for believing.
Then, having pronounced the name of God, M.Lamennais sings a hymn; and that is his demonstration! This first hypothesis admitted, M.Lamennais follows it with a second; namely, that there are three persons in God.

But, while Christianity teaches the dogma of the Trinity only on the authority of revelation, M.
Lamennais pretends to arrive at it by the sole force of argument; and he does not perceive that his pretended demonstration is, from beginning to end, anthropomorphism,--that is, an ascription of the faculties of the human mind and the powers of nature to the Divine substance.

New songs, new hymns! God and the Trinity thus DEMONSTRATED, the philosopher passes to the creation,--a third hypothesis, in which M.Lamennais, always eloquent, varied, and sublime, DEMONSTRATES that God made the world neither of nothing, nor of something, nor of himself; that he was free in creating, but that nevertheless he could not but create; that there is in matter a matter which is not matter; that the archetypal ideas of the world are separated from each other, in the Divine mind, by a division which is obscure and unintelligible, and yet substantial and real, which involves intelligibility, &c.