Let’s take this one step at a time. First, the Immaculate Conception is the statement by the Catholic Church that Mary was conceived without Original Sin. I’ll let Pope Pius IX explain it:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin (Pius IX, Ineffabilis Deus, 1854: S 2803).
Ok, now that we know what it is, let’s talk about objections to it. Some point out that the Bible says “All have sinned” (Rom 3:23) so it is impossible that Mary was conceived without sin, much less never sinned after being born. To address this, we need to think about what the word “all” means. I’ll be the first to admit, this seems like splitting hairs, but the argument rests upon this word. It’s worth looking at.
Does “all have sinned” mean each and every person has sinned without exception? Did Jesus sin? He was a human, he was tempted like us, and yet he didn’t sin. Do unborn babies sin? Do mentally handicapped people sin? No they don’t. How is this possible?
What about this verse? In Mt 3:5-6 we see:
At that time Jerusalem, all Judea, and the whole region around the Jordan were going out to him and were being baptized by him in the Jordan River as they acknowledged their sins. (Emphasis mine)Did all of Judea go get baptized? Really? Every man, woman and child went to the Jordan to be baptized by John the Baptist? Did they carry the ones too sick to walk? Or is it more likely that “all” actually means “many” or “most”? The translation from Hebrew (as I understand, I’m not Hebrew expert or anything) is a word meaning just that, “mny”, and the word used in Matthew and Romans that’s been translated to “all” is the same.
Yes, many have sinned, but not all; we have only to look at Jesus for proof of that.
Next, let's talk about why this isn’t an “invented doctrine” even though it’s only 150 years old and where we find evidence of the Immaculate Conception in the Bible.
First, we need to understand that unwritten Sacred Traditions of the Catholic Church do not need to be explicitly stated in the Bible. Why is that? Because Catholics don’t believe in Sola Scriptura or, Scripture Alone. We believe that unwritten tradition, those traditions handed down orally from the apostles, are perfectly valid so long as they have at least an indirect reference from the Bible and, obviously, don’t contradict the Bible.
The indirect reference to the sinless state of Mary is found in Luke 1:28 where the angel Gabriel says “Hail, full of grace.” How can Mary be full of grace if she has sinned? How can she be full of grace if she had original sin? We don’t believe that’s possible. See paragraph 490 of the Catechism for more info on that.
Now let’s look at why this isn’t an “invented tradition”. Yes, Pope Pius IX did state this infallibly in 1854, but that doesn’t mean he made it up at that time.
Catholic Traditions don’t have to be formal. They can exist in unwritten form but are defined when one of two things happen; if new revelation is imparted to the Church through the Spirit (see below) or if it becomes a point of confusion or is being misrepresented. This particular Tradition has been held since the early church fathers but wasn’t defined because it was never argued, confused, or further revealed. It was held as a given that the Tabernacle (which is what Mary truly is) must be completely stainless for it is the holy of holies, as we see in the Old Testament.. As far as further revelation, let’s check out the Catechism of the Catholic Church for more on that:
“Through the centuries the Church has become ever more aware that Mary, "full of grace" through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854.” (491 CCC, See 490-492 for more context)
The statement didn’t “invent” the doctrine. It solidified it and affirmed what we already knew but hadn’t defined.
This doctrine is the foundation of other Marian dogmas and traditions such as the Assumption and the Coronation of Mary as Queen of Heaven. I encourage you to do some more homework on this because a firm understanding of the Immaculate Conception really helps explain the rest of our beliefs regarding Mary.