In recent years, I have heard about and been asked about “other gospels.” Someone will ask about some “discovered” book/letter that completely changes how we view Jesus and the rest of the story of the Bible. Or, I have been asked, “What if someone could/did find another letter of Paul and it was provable?” This is a question of the sufficiency of Scripture.
Wayne Grudem gives this definition of the sufficiency of Scripture:
The sufficiency of Scripture means that Scripture contained all the words of God He intended His people to have at each stage of redemptive history, and that it now contains everything we need God to tell us for salvation, for trusting Him perfectly, and for obeying Him perfectly. 
Blessed are those whose way is blameless, who walk in the law of the LORD!” –Psalm 119:1
Everything that God wants us to know in order to live “blameless” lives is found in His word. Since the scripture is sufficient, we should earnestly search it to try and find what God would have us to do in a given situation.
It is true that the Bible does not speak exactly to every issue that one might face. However, it does give us enough information to make wise decisions so that we can live blamelessly and carry out “every good work” that God has placed before us (2 Timothy 3:17).
There is a temptation to try and make Scripture speak to a topic that is really not there. Many people are guilty of taking scripture out of context to support their own position. This is a poor way to make an argument. It would be better to say, “You know what? Scripture doesn’t speak to that topic directly, so what I will do is share scripture that is related to that topic, and we can ask God to guide us in our conclusion.” As a silly illustration, we can look at church worship. One church says they will meet at sunrise, another will meet at 10:30 am. Which one is correct scripturally? Well, the Bible does not say much about church meeting times (other than that we should meet–Hebrews 10:25). So is one church right and the other wrong? NO! We are allowed to determine worship times based on our community and the people whom we are trying to reach.
There may be some who will argue strongly for a position that is not supported scripturally. They will argue and argue and argue their position. Paul, in Romans 14, reminds us that there are weak and strong believers. Some people may struggle with certain things and those things have become sin for them (by becoming idols, or by controlling their life). Yet, for another, that same thing is not a sin, because it has not done for them the same thing that the other person struggled with. However, Paul encourages us not to allow others to stumble because of our participation in those things that are not directly prohibited by Scripture.
All that to say, we should hold up what the Bible does teach is moral, good, and right, and be content with all that God has given us in Scripture. We should not make a big deal about the things that God has not made a big deal about. However, if we are convicted through the Holy Spirit that an activity is a sin, we should hold it as a sin for ourselves.
Some will use this to say that some of their ‘borderline activities’ are OK because the scripture doesn’t have a “direct prohibition” regarding their pet topic. This does not let them off the hook. We still have to run that activity/speech through the stumbling filter. Does what I am doing, or what I am saying, cause someone else to stumble. Another great question is to ask yourself, “Will what I am getting ready to do bring a lost person closer to Christ?” This helps us think about how the world will view our actions–are they pointing toward our Christianity?
A final test comes from Philippians 4:8:
Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable–if anything is excellent or praiseworthy–think about such things.
Is what you are preparing to engage in true, noble, right, pure, lovely, admirable, excellent, or praiseworthy? If not, don’t.
Every word of God is flawless; He is a shield to those who take refuge in Him. Do not add to His words, or he will rebuke you and prove you a liar. –Proverbs 30:5-6
The doctrine of the sufficiency of Scripture reminds us that there is nothing else that we need, to know God. In fact, if there were a new revelation, or a new book/letter found, I would contend that it does not hold the same weight as scripture. Even if it was able to be proven as an original ‘autograph.’ It would likely be immensely helpful (showing more about the time and place of the Biblical writings), but would not have equal authority with the rest of the Bible.
Additionally, God is not hiding anything from us. We will not need to be concerned when we face God on the day of judgment that there will be some secret text or rule that he “springs” on us without warning. All that is required, all that will be judged, is already written in His word.
We know His will, we know how He wants us to live, we know what he wants us to avoid–His word is sufficient for us.
 Wayne Grudem, Systematic Theology: An Introduction to Biblical Doctrine. (Grand Rapids: Zondervan, 1994) 127.