[The incarnation] is a very considerable part of the glad tidings of the gospel, and which give it that name: when the angels related to the shepherds the birth of Christ, he said unto them; “Behold I bring you good tidings of great joy”, &c. (Luke 2:10,11). The whole gospel is a mystery; the various doctrines of it are the mysteries of the kingdom; the knowledge of which is given to some, and not to others; it is the mystery of godliness, and, without controversy, great; and this stands the first and principal article of it; “God manifest in the flesh” (1 Tim. 3:16). This is the basis of the Christian religion; a fundamental article of it; and without the belief of it no man can be a Christian; “Every spirit that confesseth that Jesus Christ is come in the flesh, is of God;” born of God, and belongs to him, and is on the side of God and truth; “And every spirit that confesseth not that Jesus Christ is come in the flesh, is not of God” (1 John 4:2,3).The incarnation of Christ is a most extraordinary and amazing affair; it is wonderful indeed, that the eternal Son of God should become man; that he should be born of a pure virgin, without any concern of man in it; that this should be brought about by the power of the Holy Ghost, in a way unseen, imperceptible and unknown, signified by his overshadowing; and all this in order to effect the most wonderful work that ever was done in the world, the redemption and salvation of men: it is a most mysterious thing, incomprehensible by men, and not to be accounted for upon the principles of natural reason; and is only to be believed and embraced upon the credit of divine revelation, to which it solely belongs. The heathens had some faint notions of it; at least say some things similar to it. The Brachmanes among the Indians, asserted, that Wistnavius, the second person of the trine-une god with them, had nine times assumed a body, and sometimes an human one; and would once more do the same again; and that he was once born of a virgin. Confucius, the famous Chinese philosopher, who lived almost five hundred years before Christ, it is said, foresaw that the Word would be made flesh; and foretold the year in which it would be; and which was the very year in which Christ was born: but this seems to savour too much of the tale of a Christian in later times. However, several of the deities and heroes of the heathens, Greeks and Romans, are represented as having no father. Now whatever notion the heathens had of an incarnate God, or of a divine Person born of a virgin, in whatsoever manner expressed; this was not owing to any discoveries made by the light of nature, but what was traditionally handed down to them, and was the broken remains of a revelation their ancestors were acquainted with. Otherwise the incarnation of the Son of God, is a doctrine of pure revelation . . .
John Gill, A Body of Doctrinal Divinity, book 5, chapter 1.