There are some beliefs that have followed Christianity over the centuries that did not actually originate with the Bible. Some of these beliefs slightly twist Scripture. Some are taken out of context to bolster a belief. Others are short-hand for biblical truths that need to be fleshed out more.
What are some things people commonly think are in the Bible that really aren't?
Jesus condemns homosexuality - Jesus never actually mentions homosexuality directly in the Bible. Paul does. This has led many to assume that Paul enacted tighter restrictions on sexual behavior than Jesus intended. But there's a simple explanation: Jesus wasn't confronted with homosexuality and Paul was. Jesus preached mostly to the Jews and those who lived near the Jews. Homosexuality had been roundly condemned since the time of Moses; it was not an issue in Jewish society. Paul preached to the Greeks and Romans where homosexual behavior was very common. Like any good teacher, Jesus and Paul aimed their speaking to the audience.
Jesus condones homosexuality - That doesn't mean to say that Jesus condoned homosexuality. He was quite specific that any sex outside of a marriage between a man and a woman was sin (Matthew 19:4-5; Mark 7:21). To claim otherwise is to twist the words of God to justify sinful behavior.
God won't give us more than we can handle - There is a popular contemporary song that claims God will not give us more than we can take and will not let us break. This is a gross misinterpretation of 1 Corinthians 10:13, and is not found anywhere else in the Bible. God, indeed, regularly gives us more than we can "handle," whether that means getting anxious, crying, or even death. What God promises is that we will not be exposed to any situation in which we absolutely must sin. He also promises to be present with us. This is a poignant reminder that God is much more interested in our holiness than our comfort—as well as conviction that we should be, too.
Relying on government for morality - There is a subset of the Christian church that believes Christians are responsible for bringing God's law to secular government. Some are convinced that when Christians are the primary influence over the world, Jesus will return. Others believe their country "should" be a godly country that follows the Bible and reaps God's blessing in return. The former is a misinterpretation of end times prophecy. The latter has misinterpreted a Christian's place in the world. Neither view is found in the Bible. John 18:36 says that Jesus' kingdom is not of this world; John 17:14 says His followers are not of this world. Christians are certainly called to use their influence to encourage others to obey God's word—even by enacting parallel civil laws if possible. But we are not to rely on the government for the enforcement of biblical standards.
Drinking alcohol is sin - This issue has been hashed out many times. The Bible does not prohibit anyone from drinking alcohol except for those taking a Levirate vow (i.e.: Samson, John the Baptist). The Bible does say we are not be drunk (Ephesians 5:18), and that elders (1 Timothy 3:3), deacons (1 Timothy 5:23), and respectable older women (Titus 2:3) should not drink too much wine (which some interpret as not drinking alcohol on a regular basis). There are good reasons for modern believers to abstain from alcohol if they so choose, but the Bible does not require it.
Asking Jesus into your heart/the sinners' prayer - The Bible does not tell us we must ask Jesus into our heart to be saved. Neither does it spell out a specific prayer we need to say. It is the Holy Spirit that comes to indwell a new believer, not Jesus, but asking Jesus into our hearts is a decent metaphor. It means that Jesus' words will be the source of our beliefs and the motivation of our actions. The sinners' prayer is a guideline that covers the basics of a decision to follow Christ. It is not a magic formula; it's just a convenient tool to describe to what a new believer is committing him or herself.
Slavery is sin - Kidnapping is sin (1 Timothy 1:8-10). Chattel slavery in which one person owns another is absolutely prohibited. But there are other situations that look like slavery that the Bible does not condemn. Indentured servitude, wherein someone sells their work for a period of time to pay off a debt, is not sin. Agreeing to work as a servant for room and board is not sin. In the Old Testament, sending a girl in an arranged marriage to her future family to be raised until she was old enough to marry was not sin. None of these things were ideal, and such servants were to seek their freedom if possible (1 Corinthians 7:21-24), but if the servanthood was intended to provide for the poor, it was not sin.
Pacifism/all killing is sin - The unfortunate translation of the sixth commandment in the King James Version has led to a lot of confusion regarding such things as military service and self-defense. More correctly, the commandment is "You will not murder," not "kill." The Mosaic law allowed killing in self defense. Jesus, who lived under the civil law of a non-Israelite nation, taught submission to that law to the point of voluntarily submitting His right to self-defense and teaching His followers to do the same. But He did not condemn soldiers who killed under the authority of an established government, and He did not condemn self-defense against private aggressors.
Satan lives in hell and demons will torment sinners in hell - For those of us who grew up on Saturday morning cartoons, it may come as a surprise that Satan does not rule hell or lead his demons to torment those who are banished there. The Bible does not say that Satan has been to hell yet. Jude 6 says, "And the angels who did not stay within their own position of authority, but left their proper dwelling, he has kept in eternal chains under gloomy darkness until the judgment of the great day." So there are some demons in a pre-hell holding tank, perhaps those who mated with human women in Genesis 6:1-4. But hell was created for the punishment of Satan and the demons (Matthew 25:41), not as a kingdom for them to rule. Until Satan is judged and thrown into the pit forever, he spends his time between heaven (Job 1:6-12) and earth (1 Peter 5:8).
Wedding rings/unity candle/veil - There are very few western wedding practices that resemble weddings in Bible times, and the Bible does not particularly endorse any of the ceremonial customs of the wedding itself, other than that the bride and groom live together afterwards. The gift of a ring to the bride most likely started as a symbol of the dowry or dower related to the marriage. The gift of a ring to the groom was a clever marketing ploy designed by jewelry dealers. The tradition of the bride and groom lighting a single candle with each of their own is a much more recent innovation, starting in the 1900s. It's a symbol of the two becoming one, but the unity candle is not found in the Bible. The veil is found in the Bible, although it was not necessarily exclusive to weddings. Jacob was tricked into marrying Leah because she was covered with a veil (Genesis 29:21-25), but his mother wore one when she met his father (Genesis 24:65), so the veil was not unique to the wedding ceremony.
For a list of sayings that are not found in the Bible, please see the GotQuestions.org article.
Does God help those who help themselves?
Is there a specific prayer of salvation?
Is drinking alcohol a sin?
Is homosexuality a sin?
Does the Bible allow for slavery?
Truth about the Bible