"Mary was that only one who merited to be called the Mother and Spouse of God". (Sermon 208)You'll notice that the person has provided a citation - the citation makes it look authentic. But, of course, I didn't stop there.
I grabbed a copy of Augustine's "Sermons on Liturgical Seasons," since that contains the range of sermons including Sermon 208. The quotation, however, was not to be found in Sermon 208 - a sermon on the occasion of Lent.
So, I did a little more digging. Alfonso de Liguori's "Glories of Mary," provides this same quotation and gives the Latin original ("Haec est quae sola meruit Mater et Spousa vocari.") as well as a more precise citation to an appendix of the Benedictine edition of Augustine's works.
In fact, upon locating the sermon, I discovered that it is listed as Sermon 208(a) on the occasion of the Feast of the Assumption of the Blessed Virgin Mary. It's not an authentic work. There are probably a variety of ways we could prove that, but the easiest way is that it is now recognized that there was no feast of the assumption in North Africa during Augustine's lifetime. Thus, the sermon was written at some later date, and merely ascribed to Augustine.
It's sad to see that some of Rome's advocates either knowingly or unwittingly are using falsehoods to try to promote their religion. It's one reason this blog exists - to shed the light of truth on the matter. And the truth is that Augustine did not call Mary "the Mother of God," nor would he have. In his authentic works he describes Mary this way:
At that time, therefore, when about to engage in divine acts, He repelled, as one unknown, her who was the mother, not of His divinity, but of His [human] infirmity.NPNF1: Vol. VII, Tractates on John, Tractate CXIX, §1, John 19:24-30.