Thanks for the question. A very short answer comes from II Peter 1:3. He has granted us all things that pertain to life and godliness. At the end of the section in verse 10 Peter says"For in this way there will be richly provided for youan entrance into the eternal kingdom of our Lord and Savior Jesus Christ." So Scripture tells us everything that we need to know to walk pleasing to God and enter into heaven. II Timothy 3:16-17 says something similar when it tells us that Scripture is profitable for teaching, reproof, correction and training in righteousness, and then goes on to tell us that it will leave us equipped for every good work.
I like the summary applications in Wayne Grudem's Systematic Theology:
2. The sufficiency of Scripture reminds us that we are to add nothing to Scripture, and that we are to consider no other writings of equal importance with Scripture.
3. The sufficiency of Scripture tells us that God does not require us to believe anything about himself or his redemptive work not found in Scripture.
4. The sufficiency of Scripture shows us that no modern revelations from God are to be placed on a level equal to Scripture in authority.
5. With regard to living the Christian life, the sufficiency of Scripture reminds us that nothing is sin that is not forbidden in Scripture either explicitly or by implication.
6. The sufficiency of Scripture also tells us that nothing is required of us by God that is not commanded in Scripture either explicitly or by implication.
7. The sufficiency of Scripture reminds us that in our doctrinal and ethical teaching we should emphasize what Scripture emphasizes and be content with what God has told us in Scripture.
We might at first think that a passage like this focuses Scripture on the religious, without making claims on the rest of life or thought. However, in telling us how to live in a way that pleases God, Scripture ends up speaking to many areas that might not appear to many to be religious. Psychology sometimes claims to have an understanding of men's nature and motives for action, and therefore also a strong claim to speak about how we should evaluate the actions of others. However, Scripture makes claims about these things that supersede what psychology claims, because we need to believe what Scripture says in order to please God. When the question arises whether humans are naturally good or not, Scripture gives us the answer in Romans 3. If we believe something different, then I John 1 tells us that the truth isn't in us. We could find many other examples of evaluating people's behavior where we would have to chose between a psychological and biblical explanation, and we can be confident that Scripture provides us an answer we can be sure about. However, psychology also speaks to many things that Scripture doesn't clearly speak to and that don't appear to be necessary to pleasing God. As long as we Scripture as our certain source of truth, we may gain much benefit from studying psychology.
I like the summary applications in Wayne Grudem's Systematic Theology:
1. The sufficiency of Scripture should encourage us as we try to discover what God would have us to think (about a particular doctrinal issue) or to do (in a particular situation. We should be encouraged that everything God wants to teach us about that question is to be found in Scripture. This does not mean that the Bible answers all questions that we might think up, for "The secret things belong to the Lord our God." But it does mean that when we are facing a problem of genuine importance to our Christian life, we can approach Scripture with the confidence that from it God will provide us with guidance for that problem.
2. The sufficiency of Scripture reminds us that we are to add nothing to Scripture, and that we are to consider no other writings of equal importance with Scripture.
3. The sufficiency of Scripture tells us that God does not require us to believe anything about himself or his redemptive work not found in Scripture.
4. The sufficiency of Scripture shows us that no modern revelations from God are to be placed on a level equal to Scripture in authority.
5. With regard to living the Christian life, the sufficiency of Scripture reminds us that nothing is sin that is not forbidden in Scripture either explicitly or by implication.
6. The sufficiency of Scripture also tells us that nothing is required of us by God that is not commanded in Scripture either explicitly or by implication.
7. The sufficiency of Scripture reminds us that in our doctrinal and ethical teaching we should emphasize what Scripture emphasizes and be content with what God has told us in Scripture.
