I'll give you a simple answer even though I doubt it will be adequate. I'm sure you are familiar with the saying attributed to Augustine that says: It the major things - UNITY, in the minor things - LIBERTY, in all things - CHARITY (LOVE). The doctrines, the central tenets that make up what it even means to BE a Christian, are to be held by Christians and are spelled out clearly in Scripture. In areas that are not specifically spelled out in Scripture, there is liberty, meaning that what a person decides to believe about certain things is between him and God. For someone to say the Holy Spirit "led" him to deny the deity of Jesus Christ, for example, I think we can universally agree that the Holy Spirit would NOT be who is leading that person. On the other hand, if I want to go to an "R" rated movie, is that something the Bible says is a central tenet of the Christian faith? No, but I can be led to discern what is right or wrong for me in areas not spelled out in the Bible and, if I have dedicated my life to following Christ, I will be open to the leading of the Holy Spirit in those areas that affect my life. What might be wrong for me, may be right for someone else. But, again, I'm talking about those "gray" areas - NOT the doctrines of the Christian faith.
In the history of the Roman Catholic Church there are even examples where the hierarchy stated something was to be believed but then changed it later on. So, were they being led by the Holy Spirit to make those declarations and were they wrong to do so when they then changed their beliefs?
I ask again, were those folks who preached that contraception was a sin being led by the Holy Spirit and Scripture when they were preaching that doctrine or are they being led by the Holy Spirit and the Scripture now that they preach the exact opposite?
What you should really be asking is whether or not the Holy Spirit was even called upon by those "folks" to come to their decision about contraception. Not every church out there relies upon Him to formulate their statements of faith and I hardly think most churches even have a "plank" that addresses what they think everyone in their church MUST believe about the issue. Though the Roman Catholic Church does, how many of the "faithful" even follow it? Do you think that there just might be certain areas that even Catholics think the "church" has no business telling them what to do?
On the basic, fundamental doctrines to be believed in order to BE a Christian, yes, there MUST be unity and there IS though there is even disagreement on the explanations of some of them (i.e., salvation by faith apart from works). Where churches wander away from these tenets, they cease being Christian in God's eyes. So, no, a person is wrong to state the Holy Spirit is leading them to forsake the Scripturally defined doctrines of the faith because the Holy Spirit wouldn't do that - truth is absolute.
The same could be asked about divorce and several other things.
If people are guided by the Holy Spirit they do not reverse themselves or claim that "Christian Liberty" applies whenever they're caught in a contradiction. The Holy Spirit never leads anyone into a contradictory position from one day to the next or one decade to the next.
Of course, those who worship their own, Most High and Holy Self rather than Jesus Christ always claim that "Christian Liberty" permits them anything from adultery to the Zoroastrian duality doctrines they apply to the Holy Spirit.
I suppose that King Barry relies on the same sort of bogus application of "Christian Liberty" and "Charity" when he says that any infant that survives and abortion should just be left to starve or die from exposure. Apparently some people are comfortable with accepting as their beliefs whatever the king of their nation believes.
Good question, appropriate, and even ahead of the curve. ;)
The actual quote; "In necessariis unitas, in dubiis libertas, in omnibus caritas", is literally translated as "in necessary things unity; in uncertain things freedom; in everything compassion". It was never written by St. Augustine. It was in fact spoken by Marco Antonio Dominis, a 16th century Croatian apostate in support of the Reformation. The attribution to St. Augustine was yet another fabrication by John Wesley.
It does beg the question as to what is necessary and what is uncertain, though. The notion that unity is more important than communion and doctrine is only important to the many disparate factions who were more concerned with what it takes to BE Protestant and united in opposition to the Church than to BE Christian.
It does not matter the degree to which "faithful" Catholics believe or follow it. For Catholics liberty means the freedom to do what we ought, not what we want.
Peace be with you.