Cue the Led Zeppelin soundtrack . . .
The mark of a Christian is faith in Jesus Christ. See the criminal on the cross for exhibit A. He never tithed, read the Bible, took communion, spoke in tongues, got baptized, did good works, wore the right clothes, sang the right songs, etc. but he was saved by faith in Christ.
Of course, for those of us who aren’t put to death shortly after making an authentic confession then one would expect to see the sanctification process at work, resulting in transformed though still imperfect lives.
All Christians are saved and at least a little confused in the sense of not having 100.000% perfect theology. But the focus should be on the essentials, such as the divinity, humanity and exclusivity of Jesus (e.g., He is the only way to salvation), the authority of the Bible, etc.
Romans 14 and other passages address how we are to handle disputed matters. From this we can immediately infer two things (more here):
1. God knew we’d have disputed matters.
2. He gave guidance on how to handle them.
Real salvation will lead to transformed lives. But ultimately only God knows the true state of someone’s faith in Christ. It is possible to fake it to the world.
We all have more to learn, so even if we are saved we should seek to become less confused through Bible study. We should be like the Bereans and test things in light of scripture, and we should heed Paul’s command to the Thessalonians:
Acts 17:11 Now the Bereans were of more noble character than the Thessalonians, for they received the message with great eagerness and examined the Scriptures every day to see if what Paul said was true.
1 Thessalonians 5:21 Test everything. Hold on to the good.
But what of those who teach against the essentials? Are they saved and confused, or fakes? Do some fruit inspection. If people consistently line up against the Bible and don’t seem interested in being corrected, I’d move on.
2 Timothy 4:3 For the time will come when men will not put up with sound doctrine. Instead, to suit their own desires, they will gather around them a great number of teachers to say what their itching ears want to hear.
2 Corinthians 11:13-15 For such men are false apostles, deceitful workmen, masquerading as apostles of Christ. And no wonder, for Satan himself masquerades as an angel of light. It is not surprising, then, if his servants masquerade as servants of righteousness. Their end will be what their actions deserve.
Also see In essentials, unity; in non-essentials, liberty; in all things, charity and the importance of sound doctrine.