Bahnsen very briefly touches on this in his article by making the statement:
But, as an aside, I would argue that it's from this inerrant scripture the policies of the Church are established and measured against. For example, the Catholic Church states it's wrong to murder because it says so in scripture-not because a church father states it's wrong to murder or that it's "tradition" to believe murder is wrong. Abortion is wrong to the Church because the Church interprets the fetus as life and aborting it is murdering life. They would probably use the same scripture to verify their policy or, at least, point to some church father who pointed to another who pointed to another who pointed to scripture. I'm confident this is the way the Church works and that I would hope most Catholics would agree with me.
In the end it comes back to the written word of God.
The drawback to having revelation in oral form (or tradition) is that it is much more subject to various kinds of corrupting influences that stem from man’s imperfect abilities and sinful nature (e.g., lapses of memory and intentional distortion). To curb these forces, taught Kuyper, God cast His word into written form – thereby achieving greater durability, fixity, purity, and catholicity.[8] A written document is capable of universal distribution through repeated copying, and yet it can be preserved in various kinds of depositories from generation to generation. As such it can function both as a fixed standard by which to test all doctrines of men and as a pure guide to the way of life.
I agree with his intent. However, his is not the Catholic understanding of "Apostolic Tradition". We don't believe that "oral tradition" is equal to Apostolic Tradition. An example is infant baptism. Clearly, the Bible does not come down on one side or the other. It is ambiguous from the Bible alone. However, the Apostles are credited by Church Fathers who WRITE DOWN that infant baptism is "an ancient teaching from the Apostles". Considering that Christianity is a revealed religion whose doctrines and beliefs we have obtained from the Apostles, it SHOULD not matter in what format the teaching came to us - as long as we can identify its source. Note, infant baptism is a belief of the Church that is traced back - in writing - to before 200 AD.
This is not an uncommon misunderstanding of the Church's teachings. "Apostolic Teachings" are NOT "oral" teachings for all time - but rather, teachings that were not clear and explicit in Scriptures, but clarified (in writing!) by a subsequent generation of Christians who claimed that said teachings were Apostolic - and they were accepted universally as such by the Community of faith, the Church. Thus, Catholics hold to the Apostles' teachings, whether they were given in written form, or orally, and subsequently written down OUTSIDE of Sacred Scriptures.
I would argue that it's from this inerrant scripture the policies of the Church are established and measured against. For example, the Catholic Church states it's wrong to murder because it says so in scripture-not because a church father states it's wrong to murder or that it's "tradition" to believe murder is wrong. Abortion is wrong to the Church because the Church interprets the fetus as life and aborting it is murdering life.
Actually, BOTH are used by the Church in forming its moral dogmatic teachings. The Church doesn't go to ONLY the Scriptures to know that murder is sinful. The basic law is there, given by Moses, sure enough. However - and here is the heart of the matter - the TRADITION is utilized to determine what "TODAY'S" People are to follow. The "mind" of the Church is formed by the Scriptures and its CURRENT and PAST reading of it! Our faith is formed by the people of God's reading of these same Scriptures - past and present. The Tradition represents how they read it in the past. This provides continuity in the faith, something sadly lacking in Protestantism, because they have "no" tradition (they do, but it is not common, except Sola Scriptura itself). In the example you give above, the Church uses its Tradition to rule infallibly that abortion is wrong SPECIFICALLY BECAUSE the Church has ALWAYS interpreted abortion as murder. The Bible doesn't specifically talk about abortion. But the People of God, reading Scripture, considering their Tradition received, have judged that abortion IS murder. This is the Holy Spirit guiding the Church, guiding the Church Fathers and the community as a whole. Thus, WITH Apostolic Tradition, we are better able to see how abortion is and will always be a grevious sin.
And finally, the Magesterium is necessary to help interpet the Scriptures and Tradition in light of modern problems that the People of God run into - such as embroyic stem-cell research. Scripture doesn't mention it, nor does Tradition. However, by looking to our faith as it has developed, the teaching authority of the Church can determine what we TODAY are to do in considering what is the Will of God on such questions. The Scriptures are timeless and are meant to be read by peoples of all ages. However, they are also to be read in light of the People of God's paradigm, their manner of looking at the Scriptures themselves.
I'm confident this is the way the Church works and that I would hope most Catholics would agree with me.
The Church considers the Scriptures as being inspired by God and inerrant. But it is not a systematic textbook. There are a lot of matters that are ambiguous when read OUTSIDE of the Church's own understanding - such as free will or abortion.
Regards