Sola Scriptura

Sola Scriptura, or "Scripture Alone", is the belief that Scripture is the  final  and  only infallible  authority for the

Christian in matters of faith and practice. While there are other authorities, they are always fallible and the must always be tested by and submit to the Scriptures. This view differs from "Solo Scriptura" or "Nuda Scriptura" (common among Fundamentalist evangelicals), which sees the Bible as the ONLY authority in general.

Scripture and Tradition
Lutherans believe that there were two sources of authority for the first 300–400 years of the Church. Tradition would be understood as a summary of what was written in Scripture that had always been accepted by the universal (catholic) Church. However, unlike Rome and the East, Tradition, while authoritative is not infallible.

At this time, Scripture was in the process of being recognized (canonized) and the teachings of the apostles which had been passed on through word of mouth (tradition) was only reliable for the first 100 years (or so) of Church history. The majority of Scripture (Gospels, Acts, and Pauline corpus which makes up at least 80 percent of the NT) was accepted as authoritative by A.D. 200, if not earlier. At the same time, the teachings of the apostles that were being passed on through word of mouth was becoming increasingly obscure and unreliable. Once the New Testament had been circulated throughout the Church, and once the canon had been recognized, the Church became totally reliant upon the Scriptures (both Old and New Testaments) for  ultimate  authority in all matters of faith and practice. Scripture is always to be interpreted according to the accepted, albeit fallible,  regula fidei  of the early church as represented in the early creeds and councils.

Observed Traditions
Lutheranism does recognize many Traditions of the Early Church. For example Lutheranism accepts the teachings of the first 4-5 ecumenical councils of the undivided Catholic Church:

Ecumenical Councils

 * 1) The First Council of Nicaea (325) repudiated Arianism, declared that Christ is "homoousios with the Father" (of the same substance as the Father), and adopted the original Nicene Creed; fixed Easter date; recognised authority of the sees of Rome, Alexandria and Antioch outside their own civil provinces and granted the see of Jerusalem a position of honor.
 * 2) The First Council of Constantinople (381) repudiated Arianism and Macedonianism, declared that Christ is "born of the Father before all time", revised the Nicene Creed in regard to the Holy Spirit.
 * 3) The Council of Ephesus (431) repudiated Nestorianism, proclaimed the Virgin Mary as the Theotokos ("Birth-giver to God", "God-bearer", "Mother of God"), repudiated Pelagianism, and reaffirmed the Nicene Creed.
 * 4) The Council of Chalcedon (451) repudiated the Eutychian doctrine of monophysitism; adopted the Chalcedonian Creed, which described the hypostatic union of the two natures of Christ, human and divine; reinstated those deposed in 449 and deposed Dioscorus of Alexandria; and elevated the bishoprics of Constantinople and Jerusalem to the status of patriarchates. This is also the last council explicitly recognised by the Anglican Communion.
 * 5) The Second Council of Constantinople (553) repudiated the Three Chapters as Nestorian, condemned Origen of Alexandria, and decreed the Theopaschite Formula.

Ecumenical Creeds
"main article: Three Ecumenical Creeds"Lutherans also hold to the First Three ecumenical creeds:

Apostles' Creed

I believe in God, the Father Almighty, Maker of heaven and earth. And in Jesus Christ, His only Son, our Lord, who was conceived by the Holy Spirit, born of the virgin Mary, suffered under Pontius Pilate, was crucified, died and was buried. He descended into hell. On the third day He rose again from the dead. He ascended into heaven and sits at the right hand of God the Father Almighty. From thence He will come to judge the living and the dead. I believe in the Holy Spirit, the holy catholic Church, the communion of saints, the forgiveness of sins, the resurrection of the body, and the life everlasting. Amen.

Nicene Creed

I believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible.

And in one Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all worlds, God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father; by whom all things were made; who for us men, and for our salvation, came down from heaven, and was incarnate by the Holy Ghost of the Virgin Mary, and was made man, and was crucified also for us under Pontius Pilate; He suffered and was buried; and the third day He rose again according to the Scriptures; and ascended into heaven, and sitteth on the right hand of the Father; and He shall come again with glory to judge the quick and the dead; whose kingdom shall have no end.

And I believe in the Holy Ghost, the Lord and Giver of life, who proceedeth from the Father and the Son; who with the Father and the Son together is worshiped and glorified; who spake by the Prophets. And I believe in one holy catholic and apostolic Church. I acknowledge one Baptism for the remission of sins; and I look for the resurrection of the dead, and the life of the world to come. Amen.

Athanasian Creed

Written against the Arians.

Whosoever will be saved, before all things it is necessary that he hold the catholic faith. Which faith except every one do keep whole and undefiled, without doubt he shall perish everlastingly.

And the catholic faith is this, that we worship one God in Trinity, and Trinity in Unity; Neither confounding the Persons, nor dividing the Substance. For there is one Person of the Father, another of the Son, and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost is all one: the glory equal, the majesty coeternal. Such as the Father is, such is the Son, and such is the Holy Ghost. The Father uncreated, the Son uncreated, and the Holy Ghost uncreated. The Father incomprehensible, the Son incomprehensible, and the Holy Ghost incomprehensible. The Father eternal, the Son eternal, and the Holy Ghost eternal. And yet they are not three Eternals, but one Eternal. As there are not three Uncreated nor three Incomprehensibles, but one Uncreated and one Incomprehensible. So likewise the Father is almighty, the Son almighty, and the Holy Ghost almighty. And yet they are not three Almighties, but one Almighty. So the Father is God, the Son is God, and the Holy Ghost is God. And yet they are not three Gods, but one God. So likewise the Father is Lord, the Son Lord, and the Holy Ghost Lord. And yet not three Lords, but one Lord. For like as we are compelled by the Christian verity to acknowledge every Person by Himself to be God and Lord, So are we forbidden by the catholic religion to say, There be three Gods, or three Lords.

The Father is made of none: neither created nor begotten. The Son is of the Father alone; not made, nor created, but begotten. The Holy Ghost is of the Father and of the Son: neither made, nor created, nor begotten, but proceeding. So there is one Father, not three Fathers; one Son, not three Sons; one Holy Ghost, not three Holy Ghosts. And in this Trinity none is before or after other; none is greater or less than another; But the whole three Persons are coeternal together, and coequal: so that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshiped. He, therefore, that will be saved must thus think of the Trinity.

Furthermore, it is necessary to everlasting salvation that he also believe faithfully the incarnation of our Lord Jesus Christ. For the right faith is, that we believe and confess that our Lord Jesus Christ, the Son of God, is God and Man; God of the Substance of the Father, begotten before the worlds; and Man of the substance of His mother, born in the world; Perfect God and perfect Man, of a reasonable soul and human flesh subsisting. Equal to the Father as touching His Godhead, and inferior to the Father as touching His manhood; Who, although He be God and Man, yet He is not two, but one Christ: One, not by conversion of the Godhead into flesh, but by taking the manhood into God; One altogether; not by confusion of Substance, but by unity of Person. For as the reasonable soul and flesh is one man, so God and Man is one Christ; Who suffered for our salvation; descended into hell, rose again the third day from the dead; He ascended into heaven; He sitteth on the right hand of the Father, God Almighty; from whence He shall come to judge the quick and the dead. At whose coming all men shall rise again with their bodies, and shall give an account of their own works. And they that have done good shall go into life everlasting; and they that have done evil, into everlasting fire.

This is the catholic faith; which except a man believe faithfully and firmly, he cannot be saved.

Patristics
Patristics (or the study of the Church Fathers) is also an intense practice in Lutheranism -- as most are honored or venerated as saints (with special emphasis given to St. Augustine and those living closer to the time of Christ). By 700 A.D. all of these church fathers had died.