If so why was it heretical? Inquiring minds wanna know....

See also J S Daly's main heresies of Vatican II.

Unfortunately, most Traditional Catholics are oblivious to the FUNDAMENTAL heresy of Vatican II.  Vatican II developed a new ecclesiology where the Church consists not only of Catholics but also of those somehow invisibly united to the Church.  But most Trad Catholics hold the same core ecclesiology.

MAJOR:  There's no salvation outside the Church.
MINOR:  Heretics, schismatics, and even infidels can be saved.
CONCLUSION:  Heretics, schismatics, and infidels can be inside the Church.

Presto.  V2 ecclesiology in a nutshell.  And all the errors of Vatican II derive from this ecclesiolgy.  If you hold to the MINOR above, you have zero business rejecting Vatican II.

AUG. But you will say, your punishment compels you to confess that the end is now approaching, seeing the fulfillment of that which was foretold. For it is certain there is no country, no place in our time, which is not affected or troubled. But if those evils which mankind now suffer are sure signs that our Lord is now about to come, what means that which the Apostle says, For when they shall say peace and safety. Let us see then if it be not perhaps better to understand the words of prophecy to be not so fulfilled, but rather that they will come to pass when the tribulation of the whole world shall be such that it shall belong to the Church, which shall be troubled by the whole world, not to those who shall trouble it. For they are those who shall say, Peace and safety. But now these evils which are counted the greatest and most immoderate, we see to be common to both the kingdoms of Christ and the Devil. For the good and the evil are alike afflicted with them, and among these great evils is the yet universal resort to licentious feasts. Is not this the being dried up from fear, or rather the being burnt up from lust?

Interesting what the Saints have to say about the end times. Since many 'devout' people seeing the evils of the world would think their time is the end time. But St Augustine makes an interesting statement regarding the Church. "It shall belong to the Church, which shall be troubled by the whole world"


More 

AUG. But that the Lord may not seem to have foretold as extraordinary those things concerning His second coming, which were wont to happen to this world even before His first coming, and that we may not be laughed at by those who have read more and greater events than these in the history of nations, I think what has been said may be better understood to apply to the Church. For the Church is the sun, the moon, and the stars, to whom it was said, Fair as the moon, elect as the sun. And she will then not be seen for the unbounded rage of the persecutors. AMBROSE; While many also fall away from religion, clear faith will be obscured by the cloud of unbelief, for to me that Sun of righteousness is either diminished or increased according to my faith; and as the moon in its monthly wanings, or when it is opposite the sun by the interposition of the earth, suffers eclipse, so also the holy Church when the sins of the flesh oppose the heavenly light, cannot borrow the brightness of divine light from Christ’s rays. For in persecutions, the love of this world generally shuts out the light of the divine Sun; the stars also fall, that is, men who shine in glory fall when the bitterness of persecution waxes sharp and prevails. And this must be until the multitude of the Church be gathered in, for thus are the good tried and the weak made manifest. AUG. But in the words, And upon the earth distress of nations, He would understand by nations, not those which shall be blessed in the seed of Abraham, but those which shall stand on the left hand.