For the person who keeps all of the laws except one is as guilty as a person who has broken all of God’s laws. For the same God who said, “You must not commit adultery,” also said, “You must not murder.” So if you murder someone but do not commit adultery, you have still broken the law. (NLT)
Our God has laid down some instructions and laws and we are not at liberty to pick and choose what ones we keep and what ones we dismiss. A sin is a sin is a sin. I might pride myself on not committing adultery, but not worry too much about slandering another person through idle gossip. Obedience to God’s laws is to be thorough going.
But this does not mean all law breaking is the same. In our civil laws, the penalty for speeding is not the same as the penalty for manslaughter. So, it is with God’s laws. Jesus draws a connection elsewhere between lust and adultery. But that does not mean they are equal in terms of consequences. If I lust after someone I need to repent and deal with it. If I commit adultery, I will lose my job and cause great harm to other people. Being angry at someone will not incur civil judgement. Murdering them will.
So, on the one hand, we are not to trivialise any sort of law breaking. Such law breaking displeases God and does some sort of damage to us and others. Then again, the consequences of law breaking will vary. I ought not to lust or get angry, but refraining from adultery and murder is even more necessary because of their more serious consequences.
There are no such things as little sins, but some sins have greater earthly consequences than others.