Well, I do know what I'm talking about, you still havn't given one doctrinal difference. You gave textual differences(where the same verse was in Luke) you gave one example(where you MUST take it out of context) and now this one where you are simply misreading the passage.
Eph 1:13
KJV: And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit,
NIV: And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit,
ESV: In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,
The passage teaches that we are saved after we have "head the word of truth" and "believed in him." There is nothing doctrinally wrong with the passage. The NIV isn't teaching here that you were in Christ before you were saved. I don't read the NIV, but do have a copy of it. I'm not that familiar with it's readings and don't usually take time to defend them. I'm more familiar with the KJV and the ESV, which I do believe say it better here in this passage. You can take this phrase in the NIV out of context and come to the conclusion you gave, but that would be the only reason.