The Scriptures make this distinction, such as in Rom. Have it to yourself before God. Here faith is referred to as an opinion or scruple, such as regarding dietary practices or observing of holy days. This is not the same faith that refers to the basics of the Christian religion, such as Gal. Or we can say that faith is based upon facts, particularly the facts of the gospel, while opinion is a theory about what said facts might imply, or a theology drawn from them.
Faith is based on testimony, while an opinion is a deduction drawn from that testimony.
The facts about Christ are essential, the theories about him are not. This is why theories about the millennium or speaking in tongues or the inspiration of Scripture, or such methods as missionary societies or instrumental music are only matters of opinion and not matters of faith. Where the Bible does not speak plainly there can be no faith. We can all have our opinions and preferences so long as we do not impose them upon others as matters of faith. This is the only way unity will ever be possible.