Falling away from God's truths.  God's word says there would be a falling away from His truths before Jesus' advent.

2 Thessalonians 2:1 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him, :2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand. :3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition; :4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God. :5 Remember ye not, that, when I was yet with you, I told you these things? :6 And now ye know what withholdeth that he might be revealed in his time. :7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way. :8 And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming: :9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders, :10 And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. :11 And for this cause God shall send them strong delusion, that they should believe a lie: :12 That they all might be damned who believed not the truth, but had pleasure in unrighteousness. :13 But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:

Yes, during these dark ages God's truth was changed or they thought they did.  The devil has and is using these and other changes to deceive the whole world.

Daniel 7:25 And he shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time.

Revelation 13:5 And there was given unto him a mouth speaking great things and blasphemies; and power was given unto him to continue forty and two months. :6 And he opened his mouth in blasphemy against God, to blaspheme his name, and his tabernacle, and them that dwell in heaven. :7 And it was given unto him to make war with the saints, and to overcome them: and power was given him over all kindreds, and tongues, and nations. :8 And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world.

Praise Jesus! There will be people living God's truths at Jesus' advent.

Revelation 12:9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him. :10 And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night. :11 And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death. :17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.

14:12 Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus. :13 And I heard a voice from heaven saying unto me, Write, Blessed are the dead which die in the Lord from henceforth: Yea, saith the Spirit, that they may rest from their labours; and their works do follow them. :14 And I looked, and behold a white cloud, and upon the cloud one sat like unto the Son of man, having on his head a golden crown, and in his hand a sharp sickle.

Will you received the love of the truth of God's word, even with the strong delusions that the devil is using to deceive the whole world?  Will you believe in Jesus' advent in the flesh to die for you on the cross?  Will you believe in Jesus' advent to take His people home?  Will you believe in the Bible's Lord's day the seventh day sabbath?