During the 16th Century, there were many religious wars in England and many believers were not able to practice their faith openly, so they had to find other ways to pass on their beliefs. The song The Twelve Days of Christmas is how they did it. It was a device to teach Christian beliefs to children. The "true love" refers to God Himself.
The Partidge in a pear tree was Jesus Christ who died on a"tree" as a gift from God.
Two turtle doves were the Old and New Testaments, another gift from God.
Three French hens represented faith, hope and love, three gifts mentioned in I Corinthians 13.
Four calling birds were the four Gospels that sing the song of salvation through Jesus Christ.
The five golden rings were the first five books of the Bible, (The Torah), also called the books of Moses.
The six geese a-laying were the the six days of Creation.
The seven swans a'swimming were the seven gifts of the Holy Spirit found in Romans 12:6-8.
The eight maids a-milking were the eight beatitudes.
The nine ladies dancing were the nine fruits of the Holy Spirit from Galatians 5:22,23.
The ten lords a-leaping were the Ten Commandments.
The eleven pipers piping were the 11 faithful disciples.
The twelve drummers drumming were the 12 points of the Apostles Creed.
Thus, when they sang the The Twelve Days of Christmas, they were teaching their faith and the unbelieving world had no clue as to what they were doing!