In the earliest centuries of the Christian movement, heretics directed their most dangerous attacks upon the church's understanding of who Jesus is, and in what sense He is the very Son of God. Other crucial debates concerned the question of how the Son is related to the Father and the Holy Spirit. The earliest creeds and councils of the church were, in essence, emergency measures taken to protect the central core of Christian doctrine. At historic turning-points such as the councils at Nicaea, Constantinople, and Chalcedon, orthodoxy was vindicated and heresy was condemned--and these councils dealt with doctrines of unquestionable first-order importance. Christianity stands or falls on the affirmation that Jesus Christ is fully man and fully God.

The church quickly moved to affirm that the full deity and full humanity of Jesus Christ are absolutely necessary to the Christian faith. Any denial of what has become known as Nicaean-Chalcedonian Christology is, by definition, condemned as a heresy. The essential truths of the incarnation include the death, burial, and bodily resurrection of the Lord Jesus Christ. Those who deny these revealed truths are, by definition, not Christians.

The same is true with the doctrine of the Trinity. The early church clarified and codified its understanding of the one true and living God by affirming the full deity of the Father, the Son, and the Holy Spirit--while insisting that the Bible reveals one God in three persons.

In addition to the Christological and Trinitarian doctrines, the doctrine of justification by faith must also be included among these first-order truths. Without this doctrine, we are left with a denial of the Gospel itself, and salvation is transformed into some structure of human righteousness. The truthfulness and authority of the Holy Scriptures must also rank as a first-order doctrine, for without an affirmation of the Bible as the very Word of God, we are left without any adequate authority for distinguishing truth from error.

These first-order doctrines represent the most fundamental truths of the Christian faith, and a denial of these doctrines represents nothing less than an eventual denial of Christianity itself.

The set of second-order doctrines is distinguished from the first-order set by the fact that believing Christians may disagree on the second-order issues, though this disagreement will create significant boundaries between believers. When Christians organize themselves into congregations and denominational forms, these boundaries become evident.

Second-order issues would include the meaning and mode of baptism. Baptists and Presbyterians, for example, fervently disagree over the most basic understanding of Christian baptism. The practice of infant baptism is inconceivable to the Baptist mind, while Presbyterians trace infant baptism to their most basic understanding of the covenant. Standing together on the first-order doctrines, Baptists and Presbyterians eagerly recognize each other as believing Christians, but recognize that disagreement on issues of this importance will prevent fellowship within the same congregation or denomination.