In the beginning of medieval history, the general public of Europe celebrated various pagan religions, and it was acceptable for them to do this as it had been done in their homelands for many, many years.
Then came the new people with their beliefs in one male God. They did not like the fact that everyone else did not think the same way they did. These people tried very hard to integrate the new religion throughout the countryside, but had little success. For one thing, the new God had no mate, nor did he stand for the many traditions the old God of the people did. With the new God you weren't even allowed to have parties and enjoy yourself.
The new God's reputation was going right down the proverbial drain. The new God's followers brought their religion to other people by waging war. As the victors, they could impose their rules on the defeated people. These Warriors of God wore white tunics with large, blood red crosses embroidered upon them. They would leave their families for months at a time and travel to far off places to defeat the enemy and make them worship their God.
While in Persia, they came across a nasty God that was worshiped in that country, And wonders of wonders, he resembled the old God of the people of Europe. He was dark, half animal, with horns and a tail.
When they returned to Europe, they told the people that the old God was really Satan all because of the way he looked. The people really didn't understand any of this thinking. Before the crusaders left Europe, Satan had no tail and horns, now miraculously he did. The people of the new God insisted that if the population continued to worship the old God that they would really be bowing down to Satan, the king of all Evil, and so the myth began...