When and how did classical mythology subjects start to appear in art in medieval/Renaissance Europe?

Question

All early medieval Europe art objects I am aware of are either about the Christian subjects or about the history (chronicles etc.). Classical mythology was obviously known in Europe before, see e.g. What did people in 13th century England know about Greek mythology? , but it was not seen in the art (as far as I am aware) until the 15th century.

Then, sometime in the 15th century, we see the emergence of art objects (drawings, tapestries, etc.) showing antique mythology subjects, e.g. this drawing made by Jacopo Bellini in cr. 1420-1470.

When and, if known, why has antique mythology re-emerged in art? Was it because new knowledge has emerged or some restrictions have been abolished?

One of the possibilities is emerging archaeology (Ciriaco de' Pizzicolli, 1391 – 1453/55, has pioneered it in Europe), but there might be other reasons.

Answer 1

This short essay called "Mythology and Ideology in Italian Renaissance Art" is quite relevant. The key idea is that in the prosperous city-states of Italy, "Neoplatonist ideas were translated into allegorical and symbolic images: images drawn from pre-Christian mythology, and interpreted as symbols for concepts acceptable to Renaissance Christians."

However, according to the Wikipedia article on the Italian Renaissance:

There has been much debate as to the degree of secularism in the Renaissance, which had been emphasized by early 20th-century writers like Jacob Burckhardt based on, among other things, the presence of a relatively small number of mythological paintings. Those of Botticelli, notably The Birth of Venus and Primavera, are now among the best known, although he was deeply religious (becoming a follower of Savonarola) and the great majority of his output was of traditional religious paintings or portraits

Regarding The Birth of Venus Wikipedia further mentions:

As depictions of subjects from classical mythology on a very large scale they were virtually unprecedented in Western art since classical antiquity, as was the size and prominence of a nude female figure in the Birth. It used to be thought that they were both commissioned by the same member of the Medici family, but this is now uncertain.

They have been endlessly analysed by art historians, with the main themes being: the emulation of ancient painters and the context of wedding celebrations (generally agreed), the influence of Renaissance Neo-Platonism (somewhat controversial), and the identity of the commissioners (not agreed).