This argument is often circulated on the Internet and given the ever changing and massive number of pagan deities and practices over history, support for almost any argument that Christianity copied some pagan practice could be made I suspect.
Since Adonis is your favorite, the claim that Adonis was born of a virgin mother I thought worth a short examination as I was not familiar with it. Perhaps you have read The Jesus Mysteries, written by Timothy Freke and Peter: “In Syria, Adonis’ virgin mother is called Myrrh.” Not only do they provide no source for this claim, but apparently it is contradicted by most mythology sources. One writer noted the online Encyclopedia Mythica portrays the conception of Adonis as follows:
“The generally accepted version is that Aphrodite compelled Myrrha (or Smyrna) to commit incest with Theias, her father, the king of Assyria. Her nurse helped her with this trickery to become pregnant, and when Theias discovered this he chased her with a knife. To avoid his wrath the gods turned her into a myrrh tree. The tree later burst open, allowing Adonis to emerge. Another version says that after she slept with her father she hid in a forest where Aphrodite changed her into a tree. Theias struck the tree with an arrow, causing the tree to open and Adonis to be born. Yet another version says a wild boar open the tree with its tusks and freed the child; this is considered to be a foreshadowing of his death.”
A writer also noted that James Frazer, not pro Christianity, in The Golden Bough claims that Adonis was born of an act of incest too.
The Library notes:
“…this Smyrna conceived a passion for her father, and with the complicity of her nurse she shared her father’s bed without his knowledge for twelve nights. But when he was aware of it, he drew his sword and pursued her, and being overtaken she prayed to the gods that she might be invisible; so the gods in compassion turned her into the tree which they call smyrna (myrrh). Ten months afterwards the tree burst and Adonis, as he is called, was
I have also read that the earliest known record of a tradition involving a revived Adonis [annually for one day and clearly not a defeat of death but more like a seasonal plant – I’d say vastly different than an eternal resurrection] comes from Lucian of Samosata, the author of De Dea Syria. In that work, he wrote of an annual ceremony commemorating the death, or apparent death, of Adonis and that he was killed, or apparently killed, by a boar:
“They say, at any rate, that the deed that was done to Adon by the boar occurred in their land, and in memory of that misfortune every year they beat their breasts and mourn and perform the ceremonies, making solemn lamentations throughout the country. And when the breast-beating and weeping is at end, first they make offerings to Adon as if to a dead person; and then, on the next day, they proclaim that he is alive and fetch him forth into the air, and shave their heads as the Egyptians do…”
But De Dea Syria was written during the second century AD, so the New Testament could not have been influenced by De Dea Syria.
The above from only a short lunchtime internet prowl, but rebuttals of F. Cumont, Kersey Graves, and Frazer abound and seem far more grounded, but work beckons, so no time to visit C. S. Lewis today.