The Latin Church got its idea of justice from the Romans. From the time of Augustine they read into the Bible Rome's idea of justice, and ever since the Church has had problems understanding what God’s justice is really like. (The following looks closely at what we should mean when we say, “God is just.” See here. For in-depth look at God’s justice see Atonement, Justice, and Peace: The Message of the Cross and the Mission of the Church by Darrin W. Snyder Belousek. This may be the best book on atonement ever written. It is certainly one of the most well researched. )
Some people think that God's justice is best expressed by a theory of atonement called “Penal Substitutionary Atonement” (PSA). And many of those people believe that that theory of atonement is the Gospel itself. They believe that if you do not accept that particular theory of atonement (and accept another), you are not accepting the Gospel, and therefore are not saved. If this thinking is correct, many of the early Christians were not really Christians because they accepted a different theory of atonement. (One which like PSA, is true only in a figurative sense.) The following video is about the theory of atonement which much of the early Church believed.
The following are biblical criticisms of penal substitutionary atonement.
Problems with Penal Substitution - Atonement 1 of 5
Forsaking Penal Substitution, Pt. 2: A Biblical Critique
Evaluating the Cross & 'Penal Substitution'
God’s justice is nothing like man’s justice. His justice is truly just. He does not require us to accept a theory about why he died and rose from the dead to be saved. He requires us to trust him. See the sermon Justice. (There is also a very interesting discussion about the biblical view of justice in the following interview. See here.)
Penal substitutionary atonement cannot be the Gospel because a person does not have to accept it, to be saved. See here.
What's Wrong with the Innocent Dying for the Guilty?