Sometimes Catholics talk about "dogmas," and more specifically, "Marian dogmas." It can be a confusing term, especially to non-Catholics who are not familiar with the development of doctrine within the Catholic Church.
First of all, the word "dogma" is a word that comes from the Greek dogma or dokein. Initially it was used, as the Catholic Encyclopedia describes, "in the writings of the ancient classical authors, sometimes, an opinion or that which seems true to a person; sometimes, the philosophical doctrines or tenets, and especially the distinctive philosophical doctrines, of a particular school of philosophers (cf. Cic. Ac., ii, 9), and sometimes, a public decree or ordinance."
Over time the Church adopted this word to signify "a truth appertaining to faith or morals, revealed by God, transmitted from the Apostles in the Scriptures or by tradition, and proposed by the Church for the acceptance of the faithful."
Dogmas are truths of the Catholic faith that are objectively true, finding their ultimate source in God's revelation. They are doctrines of the Catholic faith that the faithful are exhorted to believe and assent to, such as the dogma that Christ is the head of the Church or that God is a unity of three persons.
Sometimes it takes the Church centuries to fully define a dogma, as Jesus did not write a book of beliefs before he ascended into Heaven. He spoke to his apostles and while they did write down the Gospels and the New Testament letters, the Church continues to examine these writings to more fully understand the faith. This understanding is aided by people asking critical questions about it, or sometimes by teaching what is recognized as "heresy" -- that is, a teaching other than what the Church knows to be true.
Four Marian dogmas
When it comes to Mary, the Church has only defined four dogmatic beliefs, in some of the cases after debating about them for hundreds of years. These beliefs have biblical basis, but were not immediately evident to anyone reading the Bible. It took years and years of debate and prayer to come to the following conclusions, with the guidance of the Holy Spirit.
In each case, the debate among the theologians also drew from what the faithful perceived and recognized as true. So, for example, though the Immaculate Conception was not dogmatically defined until the 1800s, devotion to Our Lady as the immaculate one was already rooted in centuries of prayer and piety.
1Mary, Mother of God
[T]he Council of Ephesus proclaimed in 431 that Mary truly became the Mother of God by the human conception of the Son of God in her womb: "Mother of God, not that the nature of the Word or his divinity received the beginning of its existence from the holy Virgin, but that, since the holy body, animated by a rational soul, which the Word of God united to himself according to the hypostasis, was born from her, the Word is said to be born according to the flesh." (CCC 466)
2Mary, Ever-Virgin
The deepening of faith in the virginal motherhood led the Church to confess Mary's real and perpetual virginity before, after, and in the act of giving birth to the Son of God made man. In fact, Christ's birth "did not diminish his mother's virginal integrity but sanctified it." And so the liturgy of the Church celebrates Mary as Aeiparthenos, the "Ever-virgin." (CCC 499)
3Mary, Immaculately Conceived
From among the descendants of Eve, God chose the Virgin Mary to be the mother of his Son. "Full of grace," Mary is "the most excellent fruit of redemption" (SC 103): from the first instant of her conception in her mother's womb, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life. (CCC 503)
4Mary, Assumed into Heaven
"Finally the Immaculate Virgin, preserved free from all stain of original sin, when the course of her earthly life was finished, was taken up body and soul into heavenly glory, and exalted by the Lord as Queen over all things, so that she might be the more fully conformed to her Son, the Lord of lords and conqueror of sin and death." The Assumption of the Blessed Virgin is a singular participation in her Son's Resurrection and an anticipation of the resurrection of other Christians. (CCC 966)
