In order for me to consider someone an orthodox Christian with a small o, I would look for belief in Jesus as being both true God and true man, and believing in the Trinity--Father, Son and Holy Spirit, and believing that Jesus rose bodily from the dead. Essentially, I would put forth the Nicene-Constantinopolitan Creed:
I believe in one God, Father Almighty, Creator of heaven and earth, and of all things visible and invisible.
And in one Lord Jesus Christ, the only-begotten Son of God, born of the Father before all ages. Light of Light, true God of true God, begotten not made, consubstantial 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 Spirit and the Virgin Mary and was made man. He was crucified also for us under Pontius Pilate and suffered and was buried, And the third day He arose again, according to the Scriptures. And He ascended into heaven and sits at the right hand of the Father. And He shall come again with glory to judge the living and the dead, and his kingdom shall have no end.
And in the Holy Spirit, the Lord and Giver of Life, Who proceeds from the Father, Who together with the Father and the Son is adored and glorified, Who spoke through the prophets.
In one, holy, catholic and apostolic Church. I profess one baptism for the remission of sins. I look for the resurrection of the dead, and the life of the world to come. Amen.
In my view, anyone claiming to be Christian who disputes that Jesus is truly God, or that God is Trinity, or that Jesus didn't actually rise from the dead, is quite simply an unorthodox ("unorthodox" means "not rightly believing") Christian. Those who hold to the Creed and to at least the first three or four Ecumenical Councils is an orthodox Christian. Those who waver between orthodoxy and unorthodoxy are people who are, in my view, unsure of themselves, plain and simple.