webster's defines orthodox as "conforming to established doctrine especially in religion". when someone says they are a christian, what they should mean is that they believe and affirm the most basic tenents of the christian faith as historically recognized by christians. obviously there are differences among christians. baptists, methodists, presbyterians, anglicans, etc. can all legitimately be true christians, despite their differences in certain areas. (baptism, church structure, etc.)
but certain things are not negotiable. there are some things that one cannot deny, or must affirm in order to be a christian.
one such summary of non-negotiable christian beliefs is the nicene creed. affirmed by the council of nicea in 325 a.d. , this document affirmed the core beliefs of christianity in an effort to combat the heresies of that day. in doing so, the council has recorded for us a very succinct record of non-negotiable affirmations of the christian faith.
two quick notes.
1) creeds have no authority of their own. they are only accurate insofar as they accurately reflect the teachings of scripture. having said that, creeds and confessions of faith can be very helpful in helping us learn and retain what scripture teaches.
2) the nicene creed speaks of "one holy catholic and apostolic church". the use of the word "catholic" is not a reference to roman catholicism-but rather the church universal.
the 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 his 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 both
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 worshipped and glorified;
who spake by the Prophets.
And I believe 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.