Yes and no. There are many religions that brought plant life into the home during the cold season, not just pagans. The Christmas tree is based off the celebration of plants that are evergreen year-round. In many cases, they were believed to be magical and kept evil spirits away. I remember hearing of one culture who believed that the sun got weary and sick once a year and celebrating the winter solstice brought the sun back to health. I think because Christmas and the winter solstice are literally days away from each other, it was just easier to celebrate them both at once and the two holidays became one.