...within the Church particular Churches hold a rightful place; these Churches retain their own traditions, without in any way opposing the primacy of the Chair of Peter, which presides over the whole assembly of charity (11*) and protects legitimate differences, while at the same time assuring that such differences do not hinder unity but rather contribute toward it.
-- Lumen Gentium, 13I really wanted to say "you're being cynical and narrowly informed", but you deserve a better response than that. I am not going to attempt to address the point of excommunication, other than to say excommunication is not simply "being shown the door". Being made to feel uncomfortable in a Catholic house of worship is not being excommunicated from the mother Church.
Most biblically based Christian faiths pick and choose what they like to use and what they like to disregard: the argument about what still applies of the Old Covenant after Christ came to fulfill it began in the time of Christ and continues to this day. The Catholic church has determined doctrine by councils longer than it has looked to the Bible - the first of the councils of the Church is recorded in the Bible, and Church councils determined what would eventually be the Bible that Roman Catholics know today.
The evolution of Catholic catechism is an ongoing process. Major changes are made at ecumenical councils, and it is those councils which have shaped the Church throughout its history. Some of the most recent major changes in Catholic doctrine occured as a result of Vatican II, an ecumenical council called by Pope John Paul II. These changes were seen by many as radical, such as this one promoting tolerance:
In the second chapter, titled "On the People of God", the Council teaches that God wills to save people not just as individuals but as a people. For this reason God chose the Israelite people to be his own people and established a covenant with it, as a preparation and figure of the covenant ratified in Christ that constitutes the new People of God, which would be one, not according to the flesh, but in the Spirit and which is called the Church of Christ (Lumen Gentium, 9). All human beings are called to belong to the Church. Not all are fully incorporated into the Church, but "the Church knows that she is joined in many ways to the baptized who are honored by the name of Christ, but who do not however profess the Catholic faith in its entirety or have not preserved unity or communion under the successor of Peter" (Lumen Gentium, 15) and even with "those who have not yet received the Gospel," among whom Jews and Muslims are explicitly mentioned (Lumen Gentium, 16). The idea of any opening toward Protestantism caused a major controversy among traditionalist Catholic groups.
There was also new catechism with regard to Jews, also promoting tolerance:
"True, the Jewish authorities and those who followed their lead pressed for the death of Christ; still, what happened in His passion cannot be charged against all the Jews, without distinction, then alive, nor against the Jews of today. Although the Church is the new people of God, the Jews should not be presented as rejected or accursed by God, as if this followed from the Holy Scriptures. All should see to it, then, that in catechetical work or in the preaching of the word of God they do not teach anything that does not conform to the truth of the Gospel and the spirit of Christ. Furthermore, in her rejection of every persecution against any man, the Church, mindful of the patrimony she shares with the Jews and moved not by political reasons but by the Gospel's spiritual love, decries hatred, persecutions, displays of anti-Semitism, directed against Jews at any time and by anyone.
- Nostra Ætate"
This too was controversial, even though it affirmed the earlier findings of the Council of Trent in the 16th century. In disregard for the Council of Trent's findings that Jews do not carry the blame for the death of Christ, the "Christ killer" accusation sadly did not disappear from the mouths of the Church's believers. Many seemed to prefer looking to a much earlier Council for their beliefs about Jews, even as far back as the First Council of Nicea in 325:
"… Let us, then, have nothing in common with the Jews, who are our adversaries. … avoiding all contact with that evil way. … who, after having compassed the death of the Lord, being out of their minds, are guided not by sound reason, but by an unrestrained passion, wherever their innate madness carries them. … a people so utterly depraved. … Therefore, this irregularity must be corrected, in order that we may no more have any thing in common with those parricides and the murderers of our Lord. … no single point in common with the perjury of the Jews."
The Council of Nicea is also where this came from, which as a former Catholic you will find familiar:
We believe in one God, the Father Almighty, maker of all things visible and invisible; and in one Lord Jesus Christ, the Son of God, the only-begotten of his Father, of the substance of the Father, God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father. By whom all things were made, both which be in heaven and in earth. Who for us men and for our salvation came down and was incarnate and was made man. He suffered and the third day he rose again, and ascended into heaven. And he shall come again to judge both the quick and the dead. And in the Holy Ghost. And whosoever shall say that there was a time when the Son of God was not, or that before he was begotten he was not, or that he was made of things that were not, or that he is of a different substance or essence or that he is a creature, or subject to change or conversion--all that so say, the Catholic and Apostolic Church anathematizes them.
-- Nicene Creed
You will most likely remember a different form of this. This fundamental statement of Catholic belief also changed over time.
The councils are not frequent, but have been occurring from the earliest times, and are the process by which Catholic doctrine is shaped. Clearly, Catholic doctrine does evolve radically over time, and is not based solely on a literal reading of the Bible. It cannot be, as Catholic doctrine is older than the canonical Bible itself. The Council of Jerusalem is described in Acts and is thought to have occurred between 50 and 63. From the excellent wiki on the subject:
A common interpretation is that the council was convened as the result of the disagreement within the Early Christian community between those, such as the followers of the Pillars of the Church, led by James who believed the church must observe the rules of traditional Judaism, and Paul of Tarsus, who believed there was no such necessity (see also Supersessionism, New Covenant (theology)). However, the "rules of traditional Judaism", the Halakha of Rabbinic Judaism, were still under development at this time, as the Jewish Encyclopedia article on Jesus notes: "Jesus, however, does not appear to have taken into account the fact that the Halakah was at this period just becoming crystallized, and that much variation existed as to its definite form; the disputes of the Bet Hillel and Bet Shammai were occurring about the time of his maturity."
(Interesting that the Jews were also having their councils, evolving their religious practice, at the same time. This too is an ongoing process.)
Out of that council, Simon Peter came up with this decision:
"Wherefore my sentence is, that we trouble not them, which from among the Gentiles are turned to God: But that we write unto them, that they abstain from pollutions of idols, and fornication, and things strangled, and blood.4 For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day." (Acts 15:19-21)
which explains why observant Christians today do not eat meat that has not been drained of blood after being slaughtered by a cut to the throat.
Although this could go on for a very long time - there are more than two thousand years to cover - there is one last council which ought to show that the councils are, quite literally, before the Bible as the basis of Catholic doctrine. That one is the Tridentine Council of 1545-1563, referred to in the Catholic Encyclopedia:
The idea of a complete and clear-cut canon of the New Testament existing from the beginning, that is from Apostolic times, has no foundation in history. The Canon of the New Testament, like that of the Old, is the result of a development, of a process at once stimulated by disputes with doubters, both within and without the Church, and retarded by certain obscurities and natural hesitations, and which did not reach its final term until the dogmatic definition of the Tridentine Council.
The Bible as we know it did not reach its current form until 1563, less than five hundred years ago. The contents of the Bible grew, shrank, and changed over time by the decisions of religious scholars. There have been, and are, Bibles that differ - the Ethiopian Bible was quite different from the Bible used by Roman Catholics and other Christians, although it is now the same.