I have been dating my girlfriend for quite some time now. We are very much in love and care very deeply for each other. We're both Christians, although I attend a non-denominational church of Christ congregation and she attends a Pentecostal-ish congregation.
I tried my best to show her what I know as the truth, and she said she finally agreed with all my beliefs. Unfortunately today, after her congregation's services, she came back and told me that she had changed her mind, and that she was certain that what I believed was wrong. I know that this is because of the emotional tendencies with Pentecostals and "charismatic movement" Christians. She's letting her emotions cloud her judgment, and I can't compete with that it seems. About a month ago I was so desperate that I wrote an article about Pentecostalism and it seemed to work at the time, but, alas, the effect has faded.
I really need help. I don't want to leave her because I do think it's possible to bring her closer to Christ, and I want to do everything in my power to help her. This means so much to me. Thank you.
The fact that the effect of your arguments faded tells me that she had responded to them emotionally and not out of conviction. I would suspect that she wanted to agree with you because of your relationship.
This is going to be very difficult, but for the moment I would like you to put your emotions aside and examine who she is -- not whom you hope she will one day become. Let's suppose that you two married, what do you think your home-life will be like? Will you be comfortable with a person who makes all her decisions based on her emotions to the point of ignoring facts? You see it in the realm of religion, but she does this in many other matters to an equal degree. Is that something you could live with for the next 60 years, assuming she doesn't change?
How are your kids going to be raised? Whose church will they attend? A mother has a much stronger influence over her children than the father in their younger years. How are you going to handle disagreements about religion? Can you do it without causing your children to decide that all religions are wrong? (That, unfortunately, is the most common result.)
I know, she is just your girlfriend right now, but emotionally you are drawn to her and you need to think about what will result from that emotional tug on your heart.
What you've discussed with her isn't bad. I suspect that it has been more a one-directional discussion. You give her a lot of facts, but you aren't giving her time to digest them and make them her own. So let's change your approach just a bit.
"But avoid foolish and ignorant disputes, knowing that they generate strife. And a servant of the Lord must not quarrel but be gentle to all, able to teach, patient, in humility correcting those who are in opposition, if God perhaps will grant them repentance, so that they may know the truth, and that they may come to their senses and escape the snare of the devil, having been taken captive by him to do his will" (II Timothy 2:23-26).
Think about all the points you and she disagree with and then find one or two points that are the most fundamental difference. Make a list of all the passages you can on each point and jot a note besides each to help you remember why that passage helps your point. Pick a few that you think are the clearest.
Then sit down with your girlfriend and have a Bible study. Have her (not you) read a passage and then ask her some questions. Pay attention to her reading, where she might hesitate indicates a word she doesn't understand or a point she doesn't get. The first questions should make sure she understands what she read -- that it didn't go into one ear and out the other. The later questions should make sure she understands how the verse affects her position. But listen to her answers. If she raises issues which you have a verse that answers it, mark that one as the next one to read. If you don't have a verse, admit that you need to study that issue more.
Avoid moving on to another topic until she indicates she understands the point and sees it for herself. Silence or a lack of argument is not an agreement.
If you get to a point where a verse clearly says one thing and she insists that is something opposite, then it is time to call off the study. If God can't convince her, then no one can. For example, I once called off a study when a young man read I Peter 3:21 and said, "See, it says baptism doesn't save." Stunned, I asked him for clarification and he read his footnotes which directed him to a dictionary. The Greek wasn't translated properly, he told me. He then read from his dictionary that the word meant immediate salvation. "See," he crooned, "it doesn't save." I packed up right then. "Are you telling me I'm lost?" he demanded. I told him that if he can read a passage and his own study materials and state the opposite of what he just read, then we have no basis for further discussions.
I don't know if the same will happen with your girlfriend, or if the word of God will break through, but you have to let God speak and not tell her what she ought to think.