"Everyone believes they are right, and everyone can 'prove' it by Scripture." This is one of my major complaints about modern Christianity.
Arguments about doctrine are nothing new. They've been around since the 3,000 were baptized in Acts 2. In Acts 15, we read about the Jerusalem Council, in which the question "should Gentiles be circumcised and required to obey the law of Moses?" when they turned to God and accepted Jesus as the Messiah?
I'll take baptism as an example of "proving it".
The word "baptism" in the New Testament comes from the Greek "baptizo", meaning "dip, plunge, immerse". My own church, the Church of Christ, practices baptism by immersion. Baptists do, also; and I believe Pentecostals do. I'm sure there are others of which I am not aware. Churches of Christ use Acts 2:38-39, I Peter 3:21, Romans 6:3-7, and others to "prove" that baptism is necessary, and use the Greek to "prove" that is must be by immersion.
There are others who practice baptism by sprinkling (Catholics, Lutherans, Methodists, Episcopalians, and others). Isaiah 52:15 is used to "prove" that sprinkling is okay; also, Ezekiel 36:25. (I pulled these from this website, written by a Methodist minister.)
Some use pouring (which is probably also referred to as "sprinkling"); i.e., pouring water on top of the head. The Amish, among others, do this.
If you look at the website by the Methodist minister, you can see that he uses Scripture to "prove" his points. On the other hand, I have been in studies with Church of Christ people who have also used Scripture to "prove" their points. And, anyone that wants to tell me that "the verses on sprinkling are from the Old Testament, and Christ fulfilled the Old Testament, and the church was founded on the New Testament--may I point out that, at least in the tradition of the Churches of Christ, the Old Testament is used to justify several practices? (Nadab and Abihu, an Old Testament account, is often used to explain why Churches of Christ don't do certain things.)
I could probably use other examples: instrumental music, women's roles in the church, speaking in tongues, divorce, homosexuality . . . but for me, they all boil down to one thing: Everyone has a Scripture to "prove" that they are right.
So who is?
Just my .04, adjusted for inflation.