Extrabiblical—the prefix in this case meaning “outside” from the Latin—refers to an instance or event or happenstant (singular of happenstance) that is tied in some way back to God. I suppose if you wanted to get extra picky with the etymology, you might just see “extra” as I’ve outlined above and then “biblical” as meaning “pertaining to a book”. Any book, albeit one that is the authority on whatever subject it addresses. For the case of argument, let’s assume we’re talking about “The Holy Bible”.
“I will worship toward Thy holy temple, and praise Thy name for Thy lovingkindness and for Thy truth: for Thou hast magnified Thy Word above all Thy name.” (Psalm 138:2, emphasis mine)
Jesus, when teaching about trusting the Father for the things that fathers provide uses the illustration of the “lilies of the field” and the “fowls of the air” (Matthew 6:28, 26 respectively) as proof there’s a God who provides abundantly. The psalmist takes all of Psalm 104 to detail the grandeur and majesty of the Lord, even going so far as to make mention of the “innumerable” things in the sea that “wait all upon Thee” (verses 25 and 27). The Lord truly is good and you don’t have to crack open the Good Book to know this. The beauty of it, though, is that once you do start reading it with an open heart and mind, you will begin to see a true picture of the author form.
“Thy people shall be willing in the day of Thy power, in the beauties of holiness from the womb of the morning: Thou hast the dew of Thy youth.” (Psalm 110:3)
Another implication of the word “extrabiblical” could reference a mis-management or misconstruance of something found in the Bible. Peter says “Knowing this first, that no prophecy of the scripture is of any private interpretation.” (2 Peter 1:20) The idea of taking the information contained that very definitely points to a certain thing—even something as-yet unseen or unknown—and then making it mean what we want it to to somehow ensure we’ll get out ahead, is dangerous.
“But unto the wicked God saith, What hast thou to do declare my statutes, or that thou shouldest take my covenant in thy mouth? Seeing thou hatest instruction, and castest my words behind thee.” (Psalm 50:16-17)
“Wherefore is there a price in the hand of a fool to get wisdom, seeing he hath no heart to it?” (Proverbs 17:16)
There is power in the Word of God. But even then, if one doesn’t know the “Lord Christ” (Colossians 3:24), then the sea of lies and misinformation in which we wade around day in and day out will inundate. It’s hard (impossible) to keep one’s head above that tide. And if we think we can get through this life without the “Spirit of Truth” (See 1 John 4:6), we are supremely mistaken.
Even things of the interior, while beautiful and wondrous and remarkable will lead us astray if we don’t take them to the Lord for appraisal. Say you had a dream full of ambiguous and obscure symbolism and action and you awoke with a mixed feeling akin to what John experienced in Revelation (Revelation 10:10): “and it was in my mouth sweet as honey: and as soon as I had eaten it, my belly was bitter.” What would you do? Would you dismiss the evident message-from-God as heresy, something from the enemy? Wait a minute before you do that and pray. Understand the God that Jesus talks about in the parables referenced up top. There are things that He tells us that maybe aren’t meant for us or about us. Take every thought captive to the obedience of Christ (2 Corinthians 10:5) and He’ll sift through, not just lies and truth, but meaning. The Lord is still speaking to this day and He wants to speak to (and through) you. But it’s not one-sided. He wants to hear your thoughts on the matter.
In closing, look at Peter. He had gone through all he had with Jesus—we know his story—and it would seem that he went back to some of the old ways of thinking that he had, for lack of a better term, waded around in during his time before meeting Christ. But that’s okay. The Holy Spirit is still speaking. In the tenth chapter of Acts (verses 10-17), the Holy Spirit visits him with a vision (He has to show Peter “thrice”; verse 16), the vision that would forever solve Peter’s adherence to the old ways of the Law of Moses in favor of the Grace offered through Christ (see John 1:17). This is serious! The Holy Spirit continues to clean out Peter’s mind and thinking to be more in tune with Heaven and then look what it says in verse 17: “Now Peter doubted in himself what this vision which he had seen should mean…” Peter is still dealing with an old, dusty, cobweb-filled thought process as to what the pure word of God really sounds like. And remember, the Holy Spirit doesn’t speak anything to us that isn’t directly from the throne of God (see John 16:3) and intended to bless and help us as we serve Him. In the eleventh chapter, he recounts the story of this vision and says “the Spirit bade me go with them, nothing doubting (verse 12, emphasis mine). Peter was doubting. The more we live and grow and learn, the more information we process, the, not harder but more important it is to remain in the simplicity of the words of Christ and of His presence in our lives as provided by the Holy Spirit. They will speak to you—they want to, they are—but nothing they say will contradict what He’s already told you in His Word.