https://www.willemwitteveen.com/the-knights-templar-life-and-death/
The ancient Egyptians worshipped a god called Ra, who had the head of a hawk and wore the sun as a blazing disk in his crown. At the solstice, when Ra began to recover from his illness, the Egyptians filled their homes with green palm rushes, which symbolized for them the triumph of life over death.
Early Romans marked the solstice with a feast called Saturnalia in honor of Saturn, the god of agriculture. The Romans knew that the solstice meant that soon, farms and orchards would be green and fruitful. To mark the occasion, they decorated their homes and temples with evergreen boughs.
In Northern Europe the mysterious Druids, the priests of the ancient Celts, also decorated their temples with evergreen boughs as a symbol of everlasting life. The fierce Vikings in Scandinavia thought that evergreens were the special plant of the sun god, Balder.
https://www.history.com/topics/christmas/history-of-christmas-trees