I am a Christian, but I am also a highly spiritual person, open to many spiritual ideas. Although in my teens I was a self-declared Atheist, I found my way back to Christianity. When I got deeper in spirituality and the esoteric teachings and Gnosticism, I've heard more often about the term karma. And now this term became more and more popular.
But karma is not a Christian term. It has nothing to do with Christianity. Mostly, due to its origins and it being strongly linked to reincarnation. Christianity does not believe in neither of those two terms.
Image source: Does Christianity believe in Karma?
Karma is a force, a law that keeps the balance between good and evil. Therefore, according to karma, if you lived a sinful life, than you have to be punished. And that punishment usually happens in the next reincarnation. It is a some kind of "eye for an eye" law. Which makes sure that if you caused pain to someone else, then you are going to feel the exact same amount of pain.
Meanwhile, Christianity believes in a force like karma, but it is God Himself. And the sinners will not reborn to get their punishment. They will go to Hell. Although, this law is similar to karma, it is called Judgment.
Now, there are numerous similarities between karma and The Judgment. But they are also very different. Karma allows you to reincarnate and feel the pain you have caused. The you can continue to reincarnate and have a normal life, without punishment. But if you are a Christian and you are faced with The Judgment, then you don't get a second chance. If you sinned, then you will end up in Hell. There is no way out. There is not any chance to correct your actions.
If you are a Christian, this post might scare you. But it should motivate you. We only live one life, according to our religion (although I am open to many other ideas). So try to live a life driven by love toward others.
"And now these three remain: faith, hope and love. But the greatest of these is love."