Two things that you should know up front:
#1 as someone raised Evangelical – I love Christmas.  Now that I am working in a Mainline church – I am warming up to Advent.
#2 I am not a Catholic Scholar so what I am about to say is my own little theory about coming to this party late.

 Do people know that the immaculate conception wasn’t Jesus?  Because  I sure didn’t. 

It was not until I was studying Christology in Seminary that I figured out that I may have been fed some bad information.

I find it really helpful to work your way backward for this kind of stuff.

The churches of early centuries wanted to say something about Jesus being fully divine and fully human.

Working with the meta-physics they had at the time, they had to get the daddy influence out of there – since that was primarily how sin was transmitted.  So Jesus couldn’t have had an earthly father.

Subsequently there couldn’t be any sex. So you have a virginal conception.

This makes a lot of sense because if you want to say that Jesus was the ‘lamb of God who takes away the sin of the world’ then Jesus has to be ‘without sin’.

So, no dad – no sex … but there is still an earthly mother… SO if you retroactively move it backward … you have to fit Mary with an immaculate conception herself.

Immaculate conception is that Mary was given a one-time (unique) preservation from original sin being transmitted.  That immaculate conception was Mary’s conception by her earthly parents. Mary was conceived by regular sinful parents in an impure act  – just like everyone else – but UNlike everyone else, God intervened and did not allow the original sin to be transmitted.  This is not saying anything about Mary’s avoidance of personal sin later … only that she did not inherit original sin.

In working backward like this you can see how this is a somewhat necessary move in order to get the elemental purity (substance) you need in order for the formula to work out the way that you are after.

No earthly dad substance + No impure sex + a Mom who was unstained by original sin = a person who was fully human yet without sin.  

If you were playing by the rules of the meta-physical game back then – this is what you had to do in order to say what they wanted to say about Jesus.

Like I said yesterday, I don’t have a problem pointing out the function of the language the early church used in order to formulate what they were trying to convey. BUT neither do I have the need to point that they were ‘wrong’ or that their formulations would not stand up under modern scientific scrutiny.

This is an important point! We do not think about sex-sperm-semen the way that they did.
We don’t hold the same views of ‘substacne’ (ousia) that the ancients did.
We don’t conceptualize matter, particles, conception the way people in the early centuries did.
We are not playing by the same philosophical rules that governed the game the Creeds were involved with.

Two last things: 

Dale Allison points out that biblical references (Genesis 24, Galatians 3, Romans 9:7) show a view that can be charicterized as little humans being IN the seed and so we were all – in a sense – IN our great grandfather’s loins before our dad was even born.  Odd imagery I know but … pre-modern conceptions just … are what they are.

– The Bible is not one book (Biblia means library) and so these different books were written at different times. I look at them as ‘snapshots’ of the time.  The Creeds are the same way.  They are snapshots of an era.  You can’t look at a picture of your grandma from her teenage years and say “this isn’t what she looks like AT ALL!”.  It’s a snapshot of specific period.