For example, Christians believe that murder is wrong. Modern science shows that humans are alive from the moment of conception. The Church's response was to say that the Christian ban on murder applies from the moment of conception. This is a development of doctrine. You will find only limited evidence for the modern pro-life view from the early Church. In fact, the best evidence I find for a strong anti-abortion stance in what could be called Scripture comes from a 2nd century book called "The Apocalypse of Peter" (see v. 25), but that book is written by someone masquerading as St. Peter and claiming a vision from God he didn't really receive, so it's not accepted by any Christian denomination. Yet in spite of this lack of direct Scriptural evidence against abortion (which was a probably even in the 1st Century, which is something which the Apocalypse of Peter does attest to), the Christian worldview still clearly prohibits abortion. You don't need a "thou shalt not commit abortion" to know that it's wrong, and the early Church all held to the belief that once a life began in the womb, it was murder to end it. At the time, the common belief was in something called "the quickening," at which point the soul was said to enter the fetus' inanimate body, a view based on bad medicine. Even non-Catholics seem to have no problem in applying a basic Christian doctrine (thou shalt not kill) in a more complicated way (by determining that it's killing from the moment of conception) based on our enhanced understanding (in this case, better science). This sort of development of doctrine is the very thing that the Church exists for.
In John 14:26, Jesus promises that "the Helper, the Holy Spirit, whom the Father will send in my name, will teach you all things and remind you of everything that I have told you." The Catholic Church's belief is merely that the Holy Spirit performs this role still. Contrast this with "continuing revelation." Doctrines & Covenants 132:61-63 of the Church of Jesus Christ of Latter Day Saints says:
"61 And again, as pertaining to the law of the priesthood—if any man espouse a virgin, and desire to espouse another, and the first give her consent, and if he espouse the second, and they are virgins, and have vowed to no other man, then is he justified; he cannot commit adultery for they are given unto him; for he cannot commit adultery with that that belongeth unto him and to no one else.In an officially binding Declaration, President Wilford Woodruff denied that plural marriage was being taught: "We are not teaching polygamy or plural marriage, nor permitting any person to enter into its practice, and I deny that either forty or any other number of plural marriages have during that period been solemnized in our Temples or in any other place in the Territory. " The LDS Church summarizes this as follows: "In obedience to direction from God, Latter-day Saints followed this practice for about 50 years during the 1800s but officially ceased the practice of such marriages after the Manifesto was issued by President Woodruff in 1890." Of course, a shrewd mind might notice that Woodruff's own words contradict this (he claimed that they weren't even teaching polygamy), but even given the LDS' official position, you have a clear instance of new revelation. Men weren't polygamous, then God allegedly tells them to be, then they're not allowed to be anymore.62 And if he have ten virgins given unto him by this law, he cannot commit adultery, for they belong to him, and they are given unto him; therefore is he justified.63 But if one or either of the ten virgins, after she is espoused, shall be with another man, she has committed adultery, and shall be destroyed; for they are given unto him to multiply and replenish the earth, according to my commandment, and to fulfil the promise which was given by my Father before the foundation of the world, and for their exaltation in the eternal worlds, that they may bear the souls of men; for herein is the work of my Father continued, that he may be glorified."
You don't see anything like this from the papacy. Even the most corrupt of popes never said, "and the Lord saith that if the pope would like some concubines, it's all good." In fact, the most corrupt of popes took a vow of celibacy, so even monogamy was forbidden them. If you read D&C 132, you'll see a lot of really creepy "Scripture" targeted at Emma Smith, the wife that Joseph Smith was cheating on - things like "I command mine handmaid, Emma Smith, to abide and cleave unto my servant Joseph, and to none else. But if she will not abide this commandment she shall be destroyed, saith the Lord; for I am the Lord thy God, and will destroy her if she abide not in my law." To a non-believer in LDS doctrine, this looks a lot like someone using his powerful position to engage in some pretty blatant domestic abuse.
In the earlier example I provided of abortion, the issue of Tradition is critical, because the Early Church Fathers settle the question of the Christian view on abortion. Priests for Life assembled a collection of quotes on the subject, and they're fantastic. They're unambiguous, as far as I can tell. I liked the first quote provided from Tertullian. Tradition allows us to see how we get from point A in the 1st Century to point B in the 21st, to determine whether we went off-course, or whether things just progressed in a specific ordained direction over time. The obvious example is the papacy. I mentioned in an earlier post that a small business with a president, secretary, and treasurer looks a lot different than a multinational corporation with a president, secretary, and treasurer. The day-to-day jobs they perform evolve over time. So if you were to take Microsoft in the mid-80s and Microsoft at the end of the millenium, you might assume they were different companies. Having an eye for the day-to-day growth allows you to determine if it is really true or not. Someone claiming that Microsoft today is a counterfeit company that just commandeered the name would have to (a) take a stand on where and when this forgery occurred, and (b) deal with the fact that Bill Gates' successor (Steve Ballner, I think) is still at the helm. Sure, it might convince someone who had spent the last couple decades out of the country, and somehow hadn't heard of Microsoft's growth, but someone who watched the company grow over time wouldn't be fooled. Same thing with the papacy. Sure, it looks different with a billion members than with a couple hundred - how could you expect it not to? But unless you can name the first pope, or somehow provide an account of how an alien leadership came in and dominated Christianity for over a millenium, the argument isn't very convincing, particularly in light of the Church's own ability to trace its authority back to St. Peter. This why all Christians should develop a strong understanding of history. If you don't know how the Word got from the Apostles to you, you're missing out on a whole lot.
In the next post, I'll address some of the specific arguments written against Catholic tradition, using a pretty standard anti-Catholic article as the jumping point for the discussion.