I Thessalonians 5:21 instructs us to "test [prove, KJV] all things," which would include our old notions, and then "hold fast" to the good ones—the ones that pass the test. A mistake many make is to follow tenaciously the instruction of Revelation 3:11 to "hold fast to what we have" while completely ignoring the additional instructions of I Thessalonians 5:21 to test first.
Experience proves that not all that we believe is truth, even if held fast for forty years. We have to test our beliefs continually and rigorously against the only standard that counts—the Bible (Acts 5:29).
Human nature is lazy and takes the easy road at every opportunity. It will rely on human reasoning, the word of others, or tradition rather than do the hard work of studying the Bible and believing what it actually says. Human nature also will not naturally do the humbling work of allowing the Bible and its plain, unambiguous verses to prove matters rather than following humanly devised ideas. The church's history over the last few decades displays the fruits of taking doctrine for granted rather than allowing clear scriptures to guide our understanding of the truth.
Why do people have so many different opinions about what the Bible says? Generally, people come to the Bible with preconceived ideas and latch on to any scripture that seems to prove their belief. At the same time, they will ignore or make light of a clear verse that obviously contradicts their belief.
God can use this as a test to determine the true intents of the heart. Where does one's allegiance really lie? Will a person humbly submit to the clear instructions of God, allowing them to lead him or her to create a true spiritual foundation (Deuteronomy 8:2-3; Psalm 149:4)? Alternatively, will they choose instead to hold on to their preconceptions or other ideas of men—their idols (Revelation 21:8)—desperately grasping at the straws of unclear scriptures to build a shaky foundation?
When doctrinal disputes arise, if a person cannot or will not prove beliefs using clear and unambiguous scriptures, that fact should raise a red flag. Clear scriptures are a solid-rock foundation. Ambiguous scriptures, open to private interpretation, lead to a foundation of sand. Only one of these foundations will stand when storms come (Matthew 7:24-27).