The Bible tells us what things are sin. 1 John 3:4 says, “sin is the trangression of the law.” Sin is anything which breaks God’s law. To know God’s law, then you have to read the Bible. In the Bible God tells exactly what He commands people to do and what He commands people not to do.
To understand the commands of God, one has to read the Bible. The Bible was written to be understood by normal people. Scripture was not written to make sense only to those who have special training or advanced degrees. It was not written to make sense only to those who know the secret code to unlocking its hidden meanings. The Bible was written to be understood by everybody. The words do not contain hidden meanings. Work will often be necessary to understand what is being taught but the words of the Bible mean what words usually mean. Do not read it looking for hidden meanings or waiting for some internal impulse to give significance to the words.
When going to the Bible to learn God’s commands, there is a lot of territory to cover. Despite the length of the Bible, God’s laws are not difficult to understand or figure out. Two passages should guide ones thinking about sin. First, Matthew 22:37-38 says, “Thou shalt love the Lord thy God with all your heart, and with all your soul and with all your mind. This is the first and great comandment.” Most important in understanding what is sin is how we think about God. God is to be the most important in your life. Nothing- not self, desires, plans, friends, family, job or any other thing- is to be more important than God. If something displeases God or becomes more important to you than God, that thing is a sin.
Second, Romans 13:9-10, “If there be any other commandment, it is briefly summed up in this saying, namely, Thou shalt love thy neighbor as thyself. Love works no ill to his neighbor. Therefore love is the fulfilling of the law.” After considering if something is pleasing to God or is more important to you than God, sin is considering self as more important than others. If something is harmful to someone else then, that thing is certainly a sin. An important distinction needs to be made. What others like and what is actually good for them is not always the same thing. Kids like ice cream and might want to eat ice cream at every meal. They might also cry and complain and tell you how bad it makes them feel to have to eat green beans, but a good parent is going to make them eat what is good for them not what makes them feel good for a little while. That which keeps the law does what is genuinely good for others and does not do anything which harms another.
If you want to know if something is a sin ask yourself two questions, does this action show that God is most important to me and does this do any injury to another person.