It seems to be a popular belief today that St. Christopher was de-sainted by the Church. At some point they decided that he might not have existed so they booted him from the list of saints since they couldn't be sure he really was a saint. This is not true. Like all myths, it has a little bit of truth but it is almost completely false.
In 1969, the Church decided to revise its universal calendar of saints. The church calendar of saints is a list of various saints who are remembered on different days. There is a universal calendar, which contains saints who are celebrated by the entire church because they had some large effect or are very popular. There are also local and religious order calendars. These have more saints whose feast days are celebrated just in the areas of that particular calendar. For example, there are particular saints celebrated only in the United States, because they have particular significance for the church in the US, but not for other countries. Also, religious orders have calendars. The Dominicans will celebrate throughout the year many saints who were part of the Dominican order, but no one else will celebrate these saints. Thus the universal calendar only has a small fraction of all the canonized saints. When this universal calendar was revised, the Church looked at all the saints then on the calendar and decided that many of them could be omitted from the new calendar. There were several reasons. First of all, as time goes on, there are more and more saints to add to the calendar, and so they wanted to cut a few so that the calendar wouldn't get overloaded. Secondly, the ones they cut were often ones who lived in the first centuries of the church and for whom there isn't a vast historical background. Since St. Christopher, and other saints like him who were left off the new calendar, lived seventeen or eighteen hundred years ago, we don't have a lot of historical, reliable information about him. In reality, for most of those saints from the first couple hundred years of the church, unless they wrote some defense of the faith which we still have, we don't know much about them. Since the Church had to cut some, they decided to cut these saints from the new calendar. This does not mean that those persons could still not be celebrated in certain parts of the world. Rather, they just are no longer celebrated worldwide.
These people are still honored as saints and can be prayed to if a person has a devotion to them. It is no disgrace or diminishment of their sainthood to not be on the universal calendar. They are in Heaven and with God whether they are on the calendar, are canonized or are otherwise officially recognized. They have reached the goal of life and are perfectly happy. If they have been recognized by the Church as canonized, they still are recognized, even if they have been taken off the calendar. The calendar is nothing more than a list of very important saints for everyone; there are many saints canonized even today who never will be on the universal calendar because their influence is more local.
St. Christopher is not the only saint who was taken off the calendar. There were many. Perhaps some others you have heard of are St. George, St. Ursula, St. Philomena and St. Barbara. Of course there are still many people and churches named after these saints, and that is great, because they still give us great examples of holiness. They just aren't on the universal calendar any more.