Are we in the great Apostasy? No, per the Church Fathers. All of them unanimously say that before the antichrist comes, the entire world will be Catholic. (not every single person, but as a whole, every nation will be a catholic nation). This has not happened yet. Many great nations have never been Catholic (i.e. Africa, China, Asia, Middle East, etc). This period where the gospel will be accepted by all nations, is that period of peace, promised by Our Lady of Fatima, the 6th age of the Church, the 'Age of Mary'. When Russia is consecrated to Her, She will convert the muslims, the protestants, the hindus and pagans. She will unite the Orthodox and other schismatic sects back to the Church, and there will be one Faith across the whole world.
This period will be short, but it will be magnificent. Just as the Resurrected Lord was only on earth for 40 days and he worked many miracles, so this resurrection of the Church will be filled with miracles and great works. It will follow the 5th age of persecution, which started roughly 500 years ago, around 1517 with Martin Luther and the start of Protestantism. Since then, the Church has been under attack, persecuted, and (in our days) seemingly destroyed and overcome by Her enemies.
But as the prophecies say, God will grant the Church the Angelic Pope and the Holy Monarch, who will lead the West back to Christianity and (eventually) will convert the East, through Our Lady. Then as the Church Fathers say, there will be the 3rd and final Holy Roman Empire, which will rule over the Catholic whole world.
Then, after this period of peace, when the Holy Monarch has died, then his kingdom will be split into 10 parts, with 10 kings to rule. This is when the antichrist will appear on the scene, as a young man. These 10 kings will war against each other for power, as a punishment for the world's sins, as the people will fall into lukewarmness and apathy of Faith. Then, as the Apocalypse tells us, eventually the antichrist will convince 7 kings to follow him and they will unite and conquer the 3 who are are for God. Then will the antichrist have power and his rule of 3.5 years will begin.
But the great "falling away" begins when the Holy Monarch dies, as this will signal to those who are watchful that the 'Age of Mary' is over and the peace of Christ is at an end. Then begins the final 7th age of antichrist, as the Apocalypse describes.