The Apostasy began when the church organized by Jesus Christ did not function as he had created it to do so and in particular, no longer had authorized servants to advance the work of his church.
I submit that the main reason Joseph Smith went into the woods near his home in the spring 1820 was to find out which of the existing churches, including Catholicism, he should join.
Of course, he was told to join none of them, because they drew near to God and Christ with their lips, yet their hearts were far from them, teaching the commandments of men. The Apostasy clearly occurred centuries earlier, requiring a restoration (bringing back that structure that Christ had when he organized his church with apostles and prophets).
Further ---- Joseph Smith as I have noted did not go into the woods near his home in the spring of 1820 to create a church of his own making, despite how our critics firmly make clear in their false statements and views.
For the Catholics who contend the Apostasy did not occur, mainly on their view, that the gates had not prevailed on his church and the church is built on Peter ------answer this with a reply that is anything than the usual reply, making clear the Protestants have no footing to stand on because they broke from Catholicism.
The restored church, The Church of Jesus Christ of Latter Day Saints was necessary because the original church began by Jesus Christ himself years after his death had become corrupted and did not have the proper priesthood authority, which is important when acting in the name of God.
I am not arrogant in having a conviction of the truth of my church. That conviction came from the Holy Ghost.