First, you can always attempt to go back to the more fundamental question of sola scriptura and authority. Where in the Bible does it say the Bible is sufficient for all our beliefs? Most likely he’ll point to 2 Tim 3:16, which says,
“All scripture is inspired by God and is useful for teaching, for refutation, for correction, and for training in righteousness”
Note that this speaks to the inspiriation and the necessiry of scripture, but not its sufficiency. In fact, no where does the Bible teach this. It is an unbiblical assumption, and by Protestant’s own logic, cannot be believed.
Then you have the issue of authority. Protestants will say that the individual believer can read and interpret the Bible themselves, without the Church. If you ask them what is the pillar and foundation of truth, most likely they’ll say the Bible. If they do not say the Church, point them to 1 Tim. 3:15:
But if I should be delayed, you should know how to behave in the household of God, which is the church of the living God, the pillar and foundation of truth
After you show them this verse (or if they correctly answer “The Church”) ask: Which church? His individual church, most likely founded in the last 150 years. His denomination, most likely from the last 600 years? What about all the other churches that disagree with his? How can disagreement be the foundation of truth? “For God is not the author of confusion” (1 Cor 14:33, KJV) There
Now, onto the question at hand. The difficulty with showing the immaculate conception in Scripture is that there is not a place that says “Mary was conceived without original sin” just as there is not a place saying “God is one divine being in three persons, Father, Son, and Holy Spirit.”
Go back to Genesis 3. In verses 14 and 15 we read what God says to the serpent (Satan) after he goads Adam and Eve into sinning.
Then the LORD God said to the snake:
Because you have done this,
cursed are you
among all the animals, tame or wild;
On your belly you shall crawl,
and dust you shall eat
all the days of your life.f
15I will put enmity between you and the woman,
and between your offspring and hers;
They will strike at your head,
while you strike at their heel
All of the players of salvation history are present in this chapter. Here we have Adam, Eve, Jesus-the New Adam, and Mary-the New Eve. The “new” New American Bible uses the pronoun “they.” The Douay-Rheims uses “She” and most others use “he.” It is my understanding that in Hebrew, they are all correct.
So first God puts “enmity” between the Satan and “the woman.” Enmity means “The state or feeling of being actively opposed or hostile to someone or something.” It is a shallow and sexist reading to say, as some has suggested, that this is why women don’t like snakes. Further, does it seem right to say that “the woman” with whom there is enmity is merely Eve? Quite frankly, that doesn’t make a lot of sense to me.
Everyone, Protestants and Catholics alike, believe that “the offspring” of the woman is Jesus. This is sometimes called the “proto-evangelium” because it is the first prophecy of Jesus. Of whom is Jesus the offspring? Keep in mind that it is odd to refer to the offspring of a woman. If you look at various translations at bible.cc/genesis/3-15.htm, you’ll see that many other translations say “seed” rather than offspring. Does it make much sense to you to speak of the “seed” of a woman? Quick biology lesson: women don’t have seeds, men do. To me, it makes about as much sense to speak of the seed of a woman as it does to speak of a woman being a virgin mother. Hmm…what does that tell you about who “the woman” really is? In other words, if Jesus is “the seed” of the woman, it is right to conclude that the woman being referred to here is the virgin mother, the woman who needed no seed of a man to conceive a child: Mary.
This is important because of “enmity.” This enmity was something God put between Satan and Mary. There is an active opposition to Mary by Satan. You will read this a lot of the Marian theologians like St. Louis de Montfort, St. Alphonsus Ligouri, and St. Maximilian Kolbe. Mary is God’s most glorious creation, and Satan HATES her. So here’s the point of this argument: if God put such strong enmity between Satan and Mary, do you think God would give Satan even one little victory over her, namely, original sin? Heck no!
Now, there is another argument which I normally use, and I think it is simpler to explain. Mary is the Ark of the New Covenant. I think this is clear in two ways: 1) in Revelation 11-12. If you look at the end of Revelation 11, the Ark of the Covenant appears in the sky. At the beginning of Rev. 12, we see the woman giving birth to Jesus. This woman, in its literal sense, is Mary. Other theologians see her as Israel and the Church. Both of these interpretations are valid and accepted (and promoted) by the Church. But in its simplest sense, this woman is Mary (note the continuation of the word “woman” as well from Genesis, through the Gospels -cp. Jn 2-and Rev 12). 2) The Ark of the Old Covenant carried the word of God (ten commandments) and the bread of life (manna). Who carried the true Word (jn 1) and Bread (jn 6)? Mary!
How does this prove the Immaculate Conception? The old ark had to be perfectly made and was the most sacred object the Israelites had, to the extent that if they touched it, even accidentally, they would die. If the old ark which carried the inferior word and bread had to be perfect, even more so the New Ark!
I hope these two arguments, even if they don’t convince your Protestant friends, will convince you. If you haven’t read it already, I recommend reading Scott Hahn’s book “Hail Holy Queen.”