Is accidentally reading a blasphemous text a sin?
No it is not, to blaspheme is to speak with contempt about God or to be defiantly irreverent. Blasphemy is verbal or written reproach of God's name, character, work, or attributes. Blasphemy was a serious crime in the law God gave to Moses. The Israelites were to worship and obey God. In Leviticus 24:10–16, a man blasphemed the name of God. To the Hebrews, a name wasn’t just a convenient label. It was a symbolic representation of a person’s character. The man in Leviticus who blasphemed God’s name was stoned to death. Isaiah 36 tells the story of Sennacherib, king of Assyria, and his attempt to demoralize Jerusalem before he attacked. After pointing out Assyria's many victories, he says, "Who are they among all the gods of these lands, that have delivered their land out of my hand, that the Lord should deliver Jerusalem out of my hand?" Isaiah 36:20. Sennacherib committed blasphemy by assuming Israel’s God was equal to the false gods of the surrounding nations. The king of Judah, Hezekiah, points out this blasphemy in his prayer to God, in which he asks that God deliver them for the purpose of defending His own honor (Isaiah 37:4, 17). And that’s exactly what God did. "Then the angel of the Lord went forth, and smote in the camp of the Assyrians a hundred and fourscore and five thousand: and when they arose early in the morning, behold, they were all dead corpses. So Sennacherib king of Assyria departed, and went and returned, and dwelt at Nineveh." Isaiah 37:36-37 Later, Sennacherib was murdered in the temple of his god Nisroch. “And it came to pass, as he was worshipping in the house of Nisroch his god, that Adrammelech and Sharezer his sons smote him with the sword; and they escaped into the land of Armenia: and Esarhaddon his son reigned in his stead.” Isaiah 37:38 Followers of God are also responsible to make sure their behavior doesn't incite others to blaspheme God. In Romans 2:17-24, Paul scolds those who claim to be saved through the law and yet still live in sin, referencing Isaiah 52:5, Paul tells them, “God’s name is blasphemed among the Gentiles because of you” (Romans 2:24). In 1 Timothy 1:20 Paul explains that he had abandoned two false teachers to Satan so they would “be taught not to blaspheme”; thus, promulgating false doctrine and leading God’s people astray, which is also a form of blasphemy. Jesus spoke of a special type of blasphemy—blasphemy against the Holy Spirit—committed by the religious leaders of His day. The situation was that the Pharisees were eyewitnesses to Jesus’ miracles, but they attributed the work of the Holy Spirit to the presence of a demon (Mark 3:22-30). Their portrayal of the Holy Spirit as demonic was a deliberate, insulting rejection of God and was unforgivable. The most significant accusation of blasphemy was one that happened to be completely false. It was for the crime of blasphemy that the priests and Pharisees condemned Jesus (Matthew 26:65). They understood that Jesus was claiming to be God. That would, indeed, be a reproach on God's character—if it wasn't true. If Jesus were just a man claiming to be God, He would have been a blasphemer. However, as the Second Person of the Trinity, Jesus could truthfully claim deity (Philippians 2:6). The fact is, every time we do or say something that gives others a false representation of the glory, holiness, authority, and character of God, we commit blasphemy. Every time we misrepresent our position as children of God, we are damaging His reputation. Fortunately, Jesus forgives even the sin of blasphemy. Peter attacked Jesus' purpose (Matthew 16:22), Paul tried to make others blaspheme (Acts 26:9-18), and Jesus' own brothers thought He was insane (Mark 3:21). All repented, and all were forgiven.