Quite simply they can’t prove it. The Church of Christ has saying Scripture interprets Scripture, I think the CC has a similar teaching, There is a happy agreement in all of Scripture (something like that). Pulling a verse out of Scripture is useful for a Bible study group or a devotion, but it is a poor way develop doctrine. Using OSAS again, Scripture tells us disciples abandoned Jesus(John 6), an apostle betrayed Him (Luke 22, Matt. 26) those who received the gift of the Holy Spirit can lose it (Heb 6). The NT books weren’t written verse by verse, but as letters and documents. We can’t interpret one verse on its own. Take this verse for example
1John 2:27 But Christ has blessed you with the Holy Spirit. Now the Spirit stays in you, and you don’t need any teachers. The Spirit is truthful and teaches you everything. So stay one in your heart with Christ, just as the Spirit has taught you to do.
Ask a priest what that means and get back to me. I’ll do some study and formulate my answer, I think this will be fun.
The dificulty is getting people to look at Scripture objectively and not trying to defend a doctrine they have grown comfortable with. Just because someone has a PhD. or is old doesn’t make them right (who was that guy last year predicting Jesus return, Camping or something). Fear of God is the beginning of knowledge, part of that fear is admitting we may be wrong and are willing to study more and more to either solidify or refute our beliefs.
The context is the entire book which is being studied. Jesus said in John that the Father is greater than the Son and the Son can’t do anything the Father has not told him to do. Jehovah’s Witnesses point to this to deny Christ’s deity, yet the entire book of John point’s to Christ as our God and personal Savior. By reading more than cherry picked verses we see Jesus is talking about His human mission and how it was orchestrated by the Father. Jesus, in His human form is under the Father’s authority.
My first question would be, is the issue critical to salvation? Are they arguing about how often to take communion, should I observe Lent, was Mary sinless, should men have long hair, should women have short hair, did Adam have a belly button, can I watch an R-rated movie? If it’s not critical to salvation it is an opinion and doesn’t matter. If it is critical to salvation then the answer is in Scripture.