Why Are the Old and New Testaments the Only Infallible Rule for Systematic Theology?
The foundation of systematic theology is the conviction that God has spoken with absolute authority in the Scriptures. The Old Testament and New Testament together form the complete, inspired, and authoritative revelation of God. Because these 66 books are God’s self-disclosure, they serve as the church’s only infallible rule for doctrine, worship, and life.
Systematic theology organizes biblical truth into coherent categories, but those categories must arise from the text of Scripture, not from human opinion, philosophical speculation, or cultural trends. If any other source is given equal or higher authority, theology becomes vulnerable to distortion, losing the clear voice of God in favor of human voices.
1. Recognizing Scripture as God’s Self-Revelation
The Old and New Testaments are not merely religious writings; they are God’s Word. The prophets in the Old Testament repeatedly declared, “Thus says the LORD” (Jeremiah 1:9; Isaiah 1:2), making it clear that their message was not their own. Likewise, the apostles wrote under the direct inspiration of the Holy Spirit (2 Peter 1:20–21), communicating the authoritative teaching of Christ.
In systematic theology, this means that every doctrine must be rooted in what God has revealed about Himself. Since God cannot lie (Titus 1:2), His Word carries the weight of absolute truth. This makes Scripture uniquely trustworthy, unlike any other source.
2. Rejecting Human Traditions as Equal Authority
Church history demonstrates that when human traditions or councils are placed on the same level as Scripture, errors inevitably follow. Jesus rebuked the Pharisees for “making void the word of God by your tradition” (Mark 7:13).
Systematic theology may benefit from the insights of church history, creeds, and theological writings, but these must be tested against the Bible. Only the Old and New Testaments have the divine authority to serve as the final standard of truth.
3. Reflecting the Principle of Sola Scriptura
The Reformation reasserted the principle of sola scriptura—that Scripture alone is the infallible rule of faith and practice. This principle flows directly from the nature of the Bible as God’s Word and safeguards the church against drifting into teachings that contradict divine revelation.
For systematic theology, sola scriptura ensures that the categories and conclusions we form are anchored in God’s eternal truth, not in theological speculation or the mood of the age.
4. Responding to the Unity of the Old and New Testaments
One reason both the Old Testament and New Testament must be received together is that they tell a unified story of God’s redemptive plan. Jesus affirmed the authority of the Old Testament (Luke 24:44) while commissioning His apostles to write what would become the New Testament (John 14:26).
Systematic theology that ignores either covenant misses the fullness of God’s revelation. The Old Testament provides the promises, patterns, and prophetic foundation; the New Testament reveals their fulfillment in Christ and His kingdom.
5. Resisting the Influence of Cultural Shifts
Theological thinking is always done in a cultural context, but the infallible rule of the Old and New Testaments keeps systematic theology from being shaped primarily by societal trends. As Isaiah 40:8 declares, “The grass withers, the flower fades, but the word of our God will stand forever.”
This stability allows the church to proclaim timeless truth even when it is unpopular, ensuring that the Gospel remains unaltered in every generation.
6. Reaffirming the Sufficiency of Scripture
The sufficiency of Scripture means that God’s Word contains everything necessary for life and godliness (2 Peter 1:3). Systematic theology depends on this sufficiency—its goal is to summarize and systematize what God has revealed, not to add to it.
While there are questions the Bible does not answer in exhaustive detail, it provides all the principles needed to form faithful doctrine, guard against error, and guide God’s people in the mission of the Gospel.
7. Relating Doctrine to the Gospel of the Kingdom
Because the Old and New Testaments are the only infallible rule, the Gospel they proclaim is the true Gospel. Scripture reveals the good news that God has acted in Christ to redeem a people and restore creation.
Systematic theology organizes this Gospel-centered truth so that believers see how every doctrine—whether about God’s nature, salvation, the church, or the end of history—connects to the person and work of Jesus. Without Scripture as the supreme authority, this Gospel unity would be lost.
8. Reinforcing the Coherence of Biblical Truth
Scripture presents a coherent message because it has one divine Author. Although written over centuries by many human authors, the Bible speaks with harmony on all matters it addresses. Systematic theology recognizes and organizes this coherence into themes and doctrines, helping the church to see the interconnections of God’s truth.
If other sources were treated as equally infallible, the result would be competing authorities and doctrinal confusion. The unity of the Old and New Testaments prevents this fragmentation.
9. Requiring the Holy Spirit’s Illumination
Even though Scripture is the only infallible rule, understanding it rightly requires the illumination of the Holy Spirit (1 Corinthians 2:12–14). This keeps systematic theology from becoming a merely academic exercise and ensures it is a spiritual discipline grounded in prayer and humility.
The Spirit who inspired the Old and New Testaments continues to guide the church into all truth, enabling believers to grasp the depths of God’s revelation and apply it faithfully.
10. Remaining Faithful Until the End
In an age of competing voices, remaining anchored to the Old and New Testaments guards the church’s perseverance. Jesus warned that false prophets would arise and lead many astray (Matthew 24:11), but those who hold fast to His Word will endure.
Systematic theology serves this endurance by keeping doctrine consistent with God’s unchanging Word, ensuring that the church remains faithful to Christ until He returns to make all things new.
Conclusion: The Scriptures as the Church’s Unshakable Foundation
The Old and New Testaments together form the only infallible rule for systematic theology because they are God’s inspired, authoritative, and sufficient revelation. Every doctrine must be measured by them. Every theological system must be subject to them. Every believer must be shaped by them.
Without this foundation, theology becomes vulnerable to error. With it, the church stands firm, proclaiming the Gospel of Christ in every age, awaiting the day when faith becomes sight.
Bible Verses Related to the Old and New Testaments as the Infallible Rule
2 Timothy 3:16–17 – “All Scripture is breathed out by God and profitable for teaching… that the man of God may be complete.”
2 Peter 1:20–21 – “No prophecy of Scripture comes from someone’s own interpretation… men spoke from God as they were carried along by the Holy Spirit.”
Psalm 19:7 – “The law of the LORD is perfect, reviving the soul.”
Isaiah 40:8 – “The grass withers, the flower fades, but the word of our God will stand forever.”
John 14:26 – “The Holy Spirit… will teach you all things and bring to your remembrance all that I have said to you.”
Luke 24:44 – “Everything written about me in the Law of Moses and the Prophets and the Psalms must be fulfilled.”
Mark 7:13 – “Thus making void the word of God by your tradition that you have handed down.”
2 Peter 1:3 – “His divine power has granted to us all things that pertain to life and godliness.”
Titus 1:2 – “God, who never lies, promised before the ages began.”
Matthew 24:35 – “Heaven and earth will pass away, but my words will not pass away.”
