Invictus directly.
In his 22nd sermon, he directly addressed those who attributed the Nativity to Sol Invictus:
Having therefore so confident a hope, dearly beloved, abide firm in the Faith in which you are built: lest that same tempter whose tyranny over you, Christ has already destroyed, win you back again with any of his wiles, and mar even the joys of the present festival by his deceitful art, misleading simpler souls with the pestilential notion of some to whom this our solemn feast day seems to derive its honour, not so much from the nativity of Christ as, according to them, from the rising of the new sun. Such men’s hearts are wrapped in total darkness, and have no growing perception of the true Light: for they are still drawn away by the foolish errors of heathendom, and because they cannot lift the eyes of their mind above that which their carnal sight beholds, they pay divine honour to the luminaries that minister to the world.
In this sermon, Pope Leo I claims that, while the two feasts were held on the same day, they are not related.
Solar symbolism was popular with early Christian writers This is also apparent in the prayers and hymns of the Church, such as the Eastern Orthodox Troparion of the Nativity:
Thy birth, O Christ our God,
rose upon the world as the light of knowledge;
for through it those who worshipped the stars
were taught by a star to adore Thee, the Sun of Righteousness,
and to know Thee, the Sunrise from on high.
O Lord, glory to Thee.
Mosaic of Sol (the Sun) in Mausoleum M in the pre-fourth-century necropolis under St Peter’s Basilica. Some have interpreted it as representing Christ.
According to the New Catholic Encyclopedia, 1967, article on