From a purely religious perspective, Christmas is a minor holiday. There's nothing particularly "miraculous" about a baby being born; that happens every day. It's Easter - and the decidedly not everyday occurrence of a resurrection - that has historically been the "main event" in the church calendar.
Christmas is one of the three main lynchpins of the Western Church year along with Easter and Pentecost, and one of the twelve Great Feasts of the Eastern Church year. It was extremely widely and popularly celebrated in the Middle Ages. The liturgical year in the West begins on the first Sunday of Advent. Christmas is not by any stretch of the imagination a 'minor' holiday from the religious perspective, unless your definition of 'minor' is 'anything other than the absolutely most important'.
Compared to Pascha, all else pales in importance., but furthermore not all churches follow the liturgical year, especially those of the Reformed tradition. Also in Jefferson's day it wasn't celebrated as much as it was earlier or later. Many Protestants objected to Christmas as a made up holiday since there is nothing in the Bible to fix what time of year Christ was born. Also many of the festivities had nothing to do with Christ. So many looked upon Christmas as a Catholic invention intended to distract us from the true meaning of Christ and the need to celebrate him all year long instead of at special occasions.
In addition, observance of Christmas in the United States waned in the years immediately after the Revolution because most of the then traditional accompaniments were seen as English customs. A Republican such as Jefferson would have tried to divest emself as much as possible of English frippery (in favor of French frippery).
Much of our current views on Christmas were shaped not by Middle Ages customs but by the Gospel according to Dickens. The reinvention of Chrstmas that transformed it from a community-centered holiday to a family-centered holiday had not yet taken place in Jefferson's day, tho the decline in its importance that allowed it to be reinvented was at its peak then. Between A Christmas Carol and the grafting on of Germanic customs originally associated with St. Nickolas' Day, Christmas was reinvented in the early 19th century to essentially become the major holiday we know and love today.
Oh, I'm well aware of this, don't worry--including the lack of liturgical calendar in the Reformed churches, the Early Modern waning in observance of Christmas, and the differences in the way it was observed before that and after--and note that I do describe Easter as 'the absolutely most important'. It's just that the statement '[f]rom a purely religious perspective, Christmas is a minor holiday' betrays, I think, a somewhat narrow understanding of what a 'purely religious perspective' might look like.
The argument, be it lodged from a secular or Low Church Protestant direction, that the fact that we don't know what time of year Jesus was actually historically born makes the celebration of Christmas less legitimate always struck me as nonsensical or at least silly outside an I think excessively concrete, narrow understanding of what holidays and observances of this kind in general are supposed to be about.
