1. Refusing to tithe
I know that you are probably wondering why this is a sin. After all, God wants the best for me, right? And sometimes the best for me requires all of my paycheck, right? Not according to Malachi. In Malachi 3:8-12, God commands His people to bring a tithe to Him. He condemns those who fail to do so, stating very plainly that they are robbing Him of what is rightly His (v.8). God even states that His people are under a curse for refusing to tithe. He gives them a challenge: test Him by tithing, "and see if I will not open the floodgates of heaven and pour out so much blessing that there will not be room enough to store it" (v.10). As someone who tithes, I can say that, since I have begun tithing, I have never lacked what I need. I don't always get what I want, but I always have what I need. Try tithing. You might be surprised what happens.
2. Refusing to evangelize
Christ gave clear instructions to His followers about evangelizing. These instructions can be found in what is often referred to as "The Great Commission," in Matthew 28:18-20. It is clear from the context that Christ was not merely offering this as a suggestion. He expects His followers to go and make disciples. This involves both evangelizing new disciples and training those new disciples so that they become spiritually mature individuals. When we refuse to listen to Christ's command, we sin.
3. Refusing to love your enemies
Human beings have a tendency to love those who love them, and despise those who despise them. This is especially true in the day and age in which we live. People, at least in the United States, appear to have become so divided that the mention of one otherwise uncontroversial fact may result in several people yelling slanders at you. That is, unless you belong to their team. Unfortunately, I have seen even those who claim the name of Christ participate in this type of behavior. This issue is a culture issue. It is a heart issue. It is a sin issue. When we refuse to love our enemies, we commit sin. In fact, the only clear remedy to this madness is to begin to demonstrate love to our enemies. This does not mean that we have to affirm anything contrary to Christ in order to love them. Rather, understood properly, love will go out of its way to bring someone to Christ. Love will not sink to the level of its enemies. Rather, love will always take the high road, keeping the focus on Christ. If we love our enemies, we will pray for them to come to Christ. If we love our enemies, we will tell them about Christ. If we love our enemies, we will discourage them from sin. Jesus commanded us to reject this idea of loving only those who love us (Matthew 5:43-48). He wants us to go out of our way to demonstrate love to our enemies.
Did you know that worry can be sinful? In Matthew 6:25-34, Jesus commands us not to worry. He tells His disciples, "Therefore I tell you, do not worry about your life, what you will eat or drink; or about your body, what you will wear. Is not life more than food and the body more than clothes? Look at the birds of the air; they do not sow or reap or store away in barns, and yet your heavenly Father feeds them. Are you not much more valuable than they? Can any one of you by worrying add a single hour to your life?.......But seek first the kingdom and his righteousness, and all these things will be given to you as well" (6:25-27, 33). Worry demonstrates a lack of trust in God's provision. God may not always give us everything we want, but He will always provide everything that His children need. So don't worry.
5. Serving money
Let's face it: we live in a consumer culture. If we want something, we have to buy it with money. Yet Christ commands His disciples in Matthew 6:19-24 not to serve money. What does it mean to serve money? It means to make money the primary object. God is supposed to be our primary focus, but when we take that focus from Him and place it on money, we sin. Money itself is not bad. Jesus had followers who were likely both rich (Matthew) and poor (Andrew and Peter). There is nothing wrong with having money. There is something wrong with having money as your primary object. Let's make sure that we put everything in its place. Money is a commodity. It is something that can be traded for goods and services as needed. In contrast God is, well, God. He is the Creator, Ruler, and Sustainer of the universe. If anyone deserves to be called our Master, it is Him. Jesus warned His disciples that they could not serve two masters, saying, "No one can serve two masters. Either you will hate the one and love the other, or you will be devoted to one and despise the other. You cannot serve both God and money" (6:24). When it comes to God and money, let's remember to keep everything in perspective. Worship God. Use money. Don't get these two mixed up.