So, I think we are in agreement here. The written Word is not "all" that God intended.
Back up. As for the "ALL THAT HE COMMANDED THEM," the N.T. writers included all that we need to fulfill that mission in their writings to the various churches of the day. They didn't include the extraneous RC practices that we see today because......well, some are false and all are extraneous, IMO. 8 : )
It is certainly not my place to argue that Scripture is not sufficient unto salvation, but consider the following:
Where do we get our beliefs about abortion? There are many OT passages that we take to imply that abortion is murder (but for example, if life actually begins...say ... an hour before birth we are still "fearfully and wonderfully made" and He still "knew us in our mother's womb"). I believe that our understanding that life begins at conception is largely the continuing working of the Holy Spirit in the body of believers (for Catholics, through the Church itself).
This ongoing horror in our land has certainly become part of our mission and we rightly wage war against it, but was it part of Scripture?
Before you answer, consider: The Jews certainly didn't seem to take all of the OT passages that we see related to abortion as being so. If something you did caused a pregnant woman to lose her child you were penalized an amount of silver (not the punishment for murder, of course). To this day, polls indicate that the vast majority of Jews in the U.S. are pro-abortion (some polls have it as high as 80%)
Consider also that the Christian churches did not come out in lockstep opposition when Roe v. Wade was announced. In fact, it was only the Catholics and Baptists among the major denominations that immediately recognized the sin. Among those who initially failed to oppose this infanticide were several denominations that held strongly to a Sola-Scriptura viewpoint - yet they failed to see it.
Before I end this, however. I cannot lump "the Jews" in one basket like that. There are certainly large numbers of faithful (largely orthodox) Jews who recognize abortion for what it is - murder. My post is not intended to begin a debate about abortion (I suspect we pretty much agree on the topic), I intended to give an example of critical church teaching that is not found explicitly in Scripture, but grew out of it in response to the Holy Spirit (and find a topic that was more contemporary than discussing the doctrine of the Trinity, etc.)