So Scripture comes from God. But is Scripture sufficient? Put another way, are there other places of authority I can seek? Or has God spoken elsewhere and thus I can "supplement" Scripture with "new revelation"?
The Reformed Church holds to the sufficiency of Scripture. That sets the Reformed Church apart from Roman Catholicism (and Eastern Orthodoxy) on the one hand, and the Charismatics on the other hand. Roman Catholicism believes that the church, especially the "see of Peter" at Rome, has magisterial or ruling authority over the church. Eastern Orthodox holds to a similar stance except it is spread out among their patriarchs. In Charismatic churches, the issue of ongoing prophecy competes with the biblical view of prophecy as expressed in 2 Peter 1:21, and thus acquire at least some form of semi-canonical status. While not all charismatics are the same, it is clear that belief in ongoing prophecy today must logically lead to opening up the (now closed) canon of Scripture and thus clearing the path for God to speak in these new prophecies apart from Scripture.
Scripture alone is sufficient for all of faith and life. This is what is taught in 2 Timothy 3:16-17, in that Scripture is adequate (Greek artios), and it is practiced by Jesus Himself. The fact that God locates authority only in His Word logically leads to saying that Scripture is sufficient, for God did not put His authority anywhere else. Thus the claims of Rome (Roman Catholicism) and Constantinople (Eastern Orthodoxy) are at once rejected. Against the Charismatics, Hebrews 1:1-2 shows us that God's revelation has ceased and finalized in the Scriptures we have. The gift of prophecy is to proclaim God's words, and with the closing of the canon, that gift is now limited to the proclamation of God's Word from the pulpit, for God does not reveal anything new today.
Scripture is made up of 66 books and uses a wide variety of styles, genres and vocabulary. It might look like Scripture is intimidating and hard to read, and it could be hard to read and understand. Yet God who authors Scripture inspires it in order for us to understand, as the Word brings faith to us (Rom. 10:17) and equips us (2 Tim. 3:16-17). The Scriptures are at the same time easy and difficult, easy since it is made to be understood yet parts of it may be difficult. This is expressed in the Westminster Confession as follows:
All things in Scripture are not alike plain in themselves, nor alike clear unto all: yet those things which are necessary to be known, believed, and observed for salvation are so clearly propounded, and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them. (WCF 1.7)
The Scriptures are the ultimate authority for the Church, and it is thus in the Reformed Church. The Creeds and Confessions frame our doctrinal conversations, yet they too are to be derived from Scripture as our ultimate authority. The Scriptures, fully inspired, authoritative and clear, grounds us in what God actually says, and, together with the derived standards of the creeds and confessions, form the basis for the life and practice of the Reformed Church.