To begin with, any religion not based upon Jesus Christ and God's revealed Word (known to us as the Holy Bible) is wrong from the very start. God only has one plan. Any system of beliefs aside from that is incorrect from the very start. Though often misunderstood, our salvation through Jesus Christ is not based upon a 'code of conduct'. The Bible tells us that Jesus Christ abolished the handwriting of ordnances that were against us. Col 2:14 (NIV) puts it in a more understandable way, "having cancelled the written code; with its regulations, that was against us and that opposed to us; he took it away, nailing it to the cross."
While the modern Mayans have adopted Catholicism, there is still considerable confluence with their pagan native rituals. One should be very cautious about adopting any religion that only resembles Christianity. To add anything to Christianity is to pollute and subvert it.
Regarding the Mayans belief system, in the early days of its formation it adopted a polytheistic type of religion. The Mayan gods were mainly rain and agricultural deities. Often in conjunction with special celebrations or holidays, they would erect carved stelae. This practice is often referred to as Stela Cultism. By the end of the 9th century, there is evidence of a decline in this particular practice.
Around 1000 AD, the foreign Itza tribe invaded and began to introduce new ideas and practices. The new forces practiced human sacrifice and militarism. Gods were introduced that were military related and a form of phallic worship was introduced leading to erotic practices under the guise of religion.
As mentioned, since the Spanish conquest of the mid-1500s, the majority of Mayans have converted to a form of religion incorporating Catholic practices with some of their ancient cultic practices. They adjusted their theology to equate to Catholic patron saints and have come to consider the Virgin Mary to their moon-goddess. There are still practices that pay homage to the rain gods. According to Encyclopedia Britannica, "Even the cross is accorded a divine status, as both rain god and oracle."
They believe that the sun and the moon once inhabited the earth. The sun was the god of hunting and music while the moon was the goddess of childbirth and weaving. They believe that 13 heavens are arranged in layers above the earth and sit on the back of a crocodile. They also believe in 9 underworlds. Each of these layers is seen to have a deity that oversees that layer.
Funerals today are a mixture of Catholic practices in addition to native rites. The body of the deceased is washed with a maize mixture that the family members drink as a way of taking away the sins of the dead person. This is counter to the Christian belief of atonement through Jesus Christ. It is only through the atoning death of Christ on the cross that a person may be saved from the penalty of their sin.
In recent times, much attention and debate has centered on the Mayan calendar. The Mayans certainly had an uncanny system for measuring time. But, the idea that the Mayans correctly predicted the end of the earth is spurious at best. God has revealed prophecy through His Bible to the church. To think that an ancient polytheistic religion would have that kind of insight may be fascinating. But, it certainly isn't consistent with how God has communicated with His people.
Source: "Mayan Religion." Encyclopedia Britannica. 1982. Volume 11, p. 719