After the Quarantine went into effect, any extra-terrestrial entities who wished to come in contact with humans, for whatever reason, could only do so in our dreams, visions, and "imagination", but not directly, and always in such a way that any appearance could be discounted as fraud by a vast majority of other people.
From that time on, people around the world began believing in an "unseen" Supreme God. In the Middle East it was Jehovah of Judaism, from which Christianity and Islam later developed. In the Far East it was the Endless Tao or Om of Taoism and Buddhism. All without visible form. Believing in a God whom one has to imagine rather than a graven image, is of course viewed as a higher level of psychological development, as it necessitates "imagination" and abstract thinking.
When did this momentous event of entering The Quarantine Zone happen in human history? I found my early hint to this in Sitchin's "The Cosmic Code", where towards the end he speaks of a time when the local god Sin/Nannar was no longer reported as having been personally seen around the temples of the region of Haran in Anatolia, southern Turkey. This occurred around the beginning of the 6th Century BCE, and that's when I think The Quarantine began. Some very important events around the world near that time can corroborate this probable date.
The century between 600 and 500 BCE is widely viewed as probably the most significant time ever in the subsequent spiritual development of our world. Basically all of the main religions which mankind follows today began at that time. Just take a look at the following dates.
The founder of Buddhism, Prince Siddhartha Gautama Buddha, was born in 563 BCE. The founder of Confucianism, K'ung Fu-tzu or Confucius, was born in 551. The founder of Taoism, Lao-tze, was born around 600 BC. The founder of Jainism in India, Vardhamana Mahavira, was born in 599.
Coincidence? We go on further west. Zoroaster, the founder of dualistic belief which lay at the basis of Second Temple Judaism and later Christianity, is widely held to have been born in 588 BCE (known as "traditional date of Zoroaster", though disputed by some.) The famed conqueror Cyrus the Great was born in 590 BCE, and founded the Achaemenid dynasty in Persia, which turned Zoroastrianism into a state religion.
The "Babylonian Captivity" of Jews according to the Bible and other sources, began approximately in 597 BCE (eleven years before Nebuchadnezzar destroyed the First Temple) and ended in 537 when the famous "Edict of Cyrus" allowed them to return to Zion. Most of the canonical biblical books as we know them are considered to have been written during the captivity years, and earlier beliefs of Judaism had undergone a major revamping there, having been greatly influenced by Chaldean and Persian practices.
Still further west, late 6th Century saw the beginning of the Golden Age in Ancient Athens, considered to have been fully ushered in a bit later during the time of Pericles (born in 495 BCE.) It was the blossoming of the Rational School of Philosophy, which produced Socrates and his followers. Socrates himself was born in 470, still very close to the time-frame we're looking at. Our main "non-religious" doctrine of today was also founded around that time, because the philosophy of these Greek scholars paved the way for modern rational secular doctrines, which extol human reason as the main virtue and deny the existence of a Supreme Being altogether.
I think the foregoing easily shows that something Very Big was happening in the spiritual realm all over the known world during the 6th Century BCE or thereabouts. It was The Quarantine going into effect. From that time on the old "gods" could no longer be physically visible on Earth, and engage in a direct frontal conversation with mankind. Even if they remained on this planet, we now belong to a world with a different vibrationary pattern, which makes them effectively invisible, "not there".
This is not to say that "they" have stopped trying influencing us. Extra-terrestrial entities have always been here, and there's certainly much credible evidence to suggest that their presence has greatly increased in recent times. But they can only contact us either in our dreams and visions, or in such a way as can be easily discounted by someone who wasn't there. And their influence can only be subtle, behind the scenes. They cannot appear fully in the flesh, publicly demonstrating fruits of their advanced science - the only thing which all of humanity would take as positive proof of their existence. Such proof will not be forthcoming, as long as we're in The Quarantine zone. The good news is however, most mystics predict that The Quarantine period is about to end.