Syncretism is a blending of two or more religious traditions, especially through symbols and narratives.

Example: Though Jesus is estimated to have been born in February, Christmas is scheduled around the same time as the winter solstice or rebirth of the sun.

The Disciples' last meal with Christ took place as a Passover meal, which is in fact scheduled in the spring. But the rising or rebirth of Christ concurs with the death of winter and rebirth of spring.

Easter and Christmas blend Christian and pagan traditions:

Christmas:

Christian / Biblical symbols: manger, star, Magi

Pagan symbols: evergreen tree (fertility), Santa (abundance), little drummer boy?

Easter:

Christian / Biblical symbols: cross, tomb

Pagan symbols: rabbits, eggs (fertility)

Attractions / Repulsions of Syncretism

Attractions

Emphasis on symbols and narratives appeals to literary or imaginative sense essential to religious thought.

Perceiving instances of unity in different religious traditions may counteract the largely divisive effects of religious traditions or sects. (Religion may unify those who share the same beliefs but, by the same token, divide those of different beliefs.)

Syncretism may appeal to monotheistic inclinations of "one God" though with many expressions or embodiments.

Repulsions

Syncretism may be especially offensive to any religion's fundamentalists, whose fervor depends on all other religions being wrong.

Syncretism may make practitioners appear generous and inclusive, but sometimes they may appropriate another tradition's symbols or narratives for their own purposes.

Syncretism may indulge human wish or desire to see likeness or unity even in disparate elements and environments.

Outside sources: 

Wikipedia: Syncretism is the belief in the mixing of various schools of thought. It is especially associated with the attempt to merge, analogize or assert the underlying unity of several originally discrete traditions, especially in religion and mythology.

Syncretism is common in literature, music, representational art and other expressions of culture. . . .

More recent religious systems that exhibit marked syncretism include Vodun and Santeria, which analogize various Yoruba and other African gods to the Roman Catholic pantheon of saints. The larger, major world religions also have exhibited degrees of syncretism. For example, pagan Yule traditions were adopted by Christianity into its Christmas celebrations, and Roman Catholicism in Central and South America integrates a number of elements derived from indigenous cultures in those areas.

Oxford English Dictionary

1. Attempted union or reconciliation of diverse or opposite tenets or practices, esp. in philosophy or religion; spec. the system or principles of a school founded in the 17th century by George Calixtus, who aimed at harmonizing the sects of Protestants and ultimately all Christian bodies

Evangelical churches with American flags up front

Where's that in the Bible?--but anyone who's grown up in such a church can only think it's the most natural combination in the world.

Is religion "pure" and "back to basics?" Fundamentalism always tries to go that way, but never completely succeeds. (What most people call "pure" is simply what they grew up with, which is typically a mix of traditions.)

authenticity or growth, change?

syncretism in Guadalupe story