How is it that different churches teach different things?
For example, baptism:
some teachers baptism is absolutely necessary, others, unnecessary.
Some teach infants may be baptized, others only adults.
Some teach baptism has an effect on the soul, others that it is merely symbolic.
Some churches teach one can be baptized more than once, others that it is a one-time event.
That is eight different teachings on baptism. Each is “backed up” by its advocates using the Bible. How are we to know which is true?
Ignore what Churches say. Especially apostate churches with priests and popes.
The word baptism literally mean "immerse" or "immersion". Christ gave us an example...he was immersed in the river himself. There's much more than that. For example archaeologists digging close to Jerusalem’s Temple Mount have discovered nearly 100 ritual baths, or bathing pools, dating to the first century BC and the first century AD. This explains how the disciples baptized so many so quickly in Acts 2. The truth is there in scripture. The preponderance of evidence in the bible, along with jewish tradition and archoalogicial evidence, shows full immersion was the way people were baptized. Or at least how the first century church did it. Things start going off the rail soon after biblical times.
“...baptism...How do we know?”
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It is quite simple really. The answer is the grammatical-historical method of interpretation.
If you read it saying that salvation is a free gift for anyone who believes in Jesus Christ and take that to mean just what it says, then you will believe exactly the same thing that all the others believe who also believe what it says.
“Believe in the Lord Jesus Christ and thou shalt be saved.”
However if you redefine any of the key terms, you can make a new religion. Teach, for example, that the word “believe” means to do the necessary good works, it only remains to have some way of declaring which works amount to believing, and one other small thing, to have some way to explain away all those pesky “not by works” verses like Titus 3:5-7.
Beware! If you just believe that book, then you will be catigated as a “fundamentalist”, a term which presumes that there are in fact foundational doctrines to Christianity and that people who do not believe those doctrines, including salvation by grace, are not Christian.
I am answering your question as if it were sincere, which if you will permit me to be honest, I seriously doubt. I do not know you and in my culture I am expected to give you the benefit of the doubt in these things. In my 52 years of ministry, this being asked in what I thought was genuine curiosity has been quite rare. Just like “Where did Cain get his wife?” some people seem to think it is a “gotcha question” or that is what they want it to be.
The folks here on FR are the dearest and best and yet even here there are some who will twist one’s words. This you well know. How is it hard to think that people do not do the same with the Word of God? Do you really want to know the Bible? Is there no living God Who can show you what He means in His Book? If your question is sincere, then you ought to have asked God directly. If you have asked Him and gotten nothing but silence, then you need to get to the bottom of that. Nothing is more important.