Who believed in the immaculate conception? How early was it? Who didn’t believe in Mary’s immaculate conception and sinlessness? Again, some of this surprises many of us reared theologically in Protestantism. After this listing, I’ll draw my conclusions. No need to drag this out all week, as I don’t know enough about it to do that.
Who did?
Ephraem the Syrian, Camina Nisibena, 27.8.
Gregory of Nazianzen, Sermon 29.

Epiphanius of Salamis, Haeresis, 42.12.
Augustine, Nature and Grace, 36, 42.
Martin Luther, Sermon on the Day of the Conception of Mary the Mother of God.
Who didn’t?
Origen, Homily on Luke, 17:6-7.
Innocent III,
Leo I,
St. Basil, Letter 260.
St. John Chrysostom, Homily on Matthew, 44; Homily on John, 21.
Gregory I
Anselm of Canterbury, Cur Deus Homo, 2.16.
Bernard of Clairvaux, Epistle 174.
Thomas Aquinas, but with a very minor quibble (at infusion of soul, not conception).
Does this listing influence what you think of this theological idea?
Now some conclusions:
There are major problems for the doctrine of immaculate conception.
First, the Bible does not teach that the virginal conception occurred to preserve Jesus from a sinful nature. Since the Bible teaches that all inherit sin at some level (in Adam etc), Joseph and Mary inherited sin (at some level). But the Bible does not say avoiding Joseph’s sinful nature is why the virginal conception occurred. What it does say is that Mary’s pregnancy was “of the Spirit” and that it fulfills Isaiah 7:14.
The issue is that she conceived supernaturally, not the why or the how.
Second, if God could have preserved Mary from the stain of original sin by an immaculate conception, why could not God also have done it that way with Jesus. If God can do this with Mary, why not do it that way with Jesus? Why the trouble? I find no necessary logic to it (though I’ll admit to my readers like Dennis Martin that it is “fitting” if one makes other assumptions).
Third, I don’t find the criticism of an infinite regression back to Eve to be of much weight. Why? Because the teaching is that Mary, at the moment (but instantaneously after the birth — I’m not sure how to say it for the RCC) of her conception was “saved” in the sense of being preserved from the stain of original sin, there is no need to go beyond Mary to her mother.
Fourth, we can once again argue from silence that no one in the NT believed Mary was sinless since no one lets on that she was. This kind of argument can only go so far. It is possible they all believed it and never had any reason to say so. I would avoid this argument as much as possible. What you can say is that since everyone sinned, Mary must have. This is a little better, but we should admit up front that one theologian who was big on original sin and human depravity, Augustine, seems also to have thought Mary was excepted by a special act of God’s grace. So, we don’t get all that far with this argument either.
Fifth, you might argue that Mary sinned when she remonstrated with Jesus in the Temple at the end of Luke 2, or that she was dull spiritually in Mark 3:31-35, etc.. For some, this is enough evidence to tip the balance in the direction of Mary’s ordinary humanity. (I deal with this in my book so I’ll avoid that discussion for right now.) You might also argue that John, by omitting her name in John 2:11, did not think she had yet become a follower of Jesus at that point in her life. By Acts 1:14 she had become one; and surely her presence at the crucifixion leans in that direction.
In sum, I don’t find the teaching of the immaculate conception required or implicit by what is clear about Mary, Joseph, and Jesus in the NT.
I would, however, contend the following to be fair in this sort of conversation:
1. If the sinful nature is passed on genetically or physically (again, I don’t know the best way to say this), then the virginal conception could have something to do with Jesus’ not having a sinful nature.
2. If Jesus’ “flesh” comes from Mary, then there is an issue with Mary passing on her sinful nature to Jesus.
3. The immaculate conception is “fitting” in such a theological framework.
4. There are other reasons, perhaps even more fitting or compelling to RC theologians for the IC, such as “full of grace” in Luke 1, but none of them is as compelling to me as the sin nature argument advocated by Ambrose.