Catholics hold to this doctrine as an infallible and foundational part of our faith. Our Protestant brothers and sisters see it as a Catholic invention that detracts from the Lord.
Our Protestant brethren could not be more wrong.
The Holy Spirit allowed Mary to say, of Herself, that her soul magnifies the Lord and that Her spirit rejoiced in God, Her Savior. (Luke 1:46-55) That our Protestant friends fail to see the significance of the Magnificat is unfortunate.
The Bible tells us that we are saved by Grace, through faith. Grace is the gift of God that sanctifies us and enables us to carry out the work God has prepared for us. (Ephesians 2:8-10).
For a better explanation of Grace, I invite you to read this article, (parts VII and VIII in particular).
The Bible shows us over and over how particular individuals are prepared by God to do significant and extraordinary work. God gives these people the graces they need to accomplish this work. Sometimes this work means enduring hardships seemingly beyond human strength. The martyrs are the best example of this.
To understand Mary and the Immaculate Conception, is to understand that work that She would be called to do. She was to become nothing less than the Ark of the Covenant. The pure, Holy vessel that was to carry the Word of God, incarnate.
You may recall that the Ark of the Covenant was that vessel created of incorruptible (acacia) wood that carried the Word of God (the Torah), The Bread of life (The manna) and the staff of the Shepherd.
The Word of God, the Bread from Heaven and the Good Shepherd are all types of Christ. Jesus said of Himself that He was the Covenant fulfilled when He said "This is my blood, the blood of the Covenant" (Matthew 26:28).
If Jesus is the Covenant fulfilled, than Mary is the Ark of the Covenant fulfilled since the Word of God, The Bread from Heaven, The Shepherd was carried inside Her. The Old Testament Ark carried things that symbolized Christ within it but was made so holy by them that for a man to even touch it meant death (2 Samuel 6:6-7).
This Ark was only a symbol! It points forward to the real Ark, which we see in Revelation 11:19- 12:5 as Jesus Mother, Mary. She carried the actual God-Man within her. Thus, She would be sanctified and made holy far more than the box that pointed towards her.
To be prepared for this, she would be sanctified by perfect, overflowing Grace from the very start of her existence- Her conception.
Our Protestant brothers and sisters mistakenly believe that Catholics have deduced this by human reason alone, without scriptural support. This is an understandable conclusion since the Church did not formally define this doctrine until 1856.
The biggest contribution to the formation of the doctrine is what we have come to understand and apply about what the scriptures say in Luke's gospel. We can also see from history how hard the devil has tried to disguise this sacred truth. That the doctrine was formally defined in 1854 does not mean it is not what the church has believed since it's very inception. In fact, the belief in Mary's Immaculate conception goes back to the very start of Christianity.
To understand this, we must understand what the Angel Gabriel says to Mary in Luke 1:28.
Bible versions such as the King James version, deliberately mistranslated the passage to appease protestants who felt Catholics were elevating Mary. This is a very dangerous thing to do. The translation of a passage should not be subject to theological whims but what the passage actually says. This is especially true for a Biblical version such as the KJV that purports to be a literal, rather than dynamic translation.
The Greek language of Luke 1:28 in no way supports the KJV's rendering of "highly favored one".
εἰσελθὼν χαῖρε κεχαριτωμένη κύριος
This passage reads eiselthōn chaire kecharitōmenē kurios. The literal translation of this passage is "and entering in, He said to Her, 'Hail thou who art endured with perfected Grace, the Lord is with you'".
The Protestant rendering of "hail, highly favored one, makes a mockery of the very next verse;
29 But she was greatly troubled at the saying, and considered in her mind what sort of greeting this might be.
What could be troubling or difficult with the message that you have are favored by God? Nothing. That is not what the passage says. Kecharitomene is a perfect, past participle of the term Charis. Charis is translated as Grace by the KJV more than 90% of the time but is translated here as favor at a time when it could not possibly be plausibly translated as favor.
For Kecharitomene not only means Grace, it means perfected grace that has existed from the start. In fact, Kecharitomene only appears 1 other time in the New Testament. In that instant, John 1:42, it refers to Jesus and is correctly translated by the KJV as Grace. Why does the KJV translate the same word two different ways? Why was it the first version to do so? An agenda?
You better believe it.
In fact, in the margin of the original King James Version, is the admission that Kecharitomene means "endured with grace". To translate it any other way is to do deliberate violence to the word.
In the very first Bible ever produced (The Vulgate) Kecharitomene is translated into the latin Gratia Plenia. In the very first English version- the Catholic Douai Rheems- it is translated- correctly- as Full of Grace.
This correct translation confirms- as Biblical fact- that Mary was indeed preserved in Grace from the moment of Her conception. The Catholic Church stated this as infallibly true in 1856 and Our Blessed Mother confirmed it to Saint Bernadette Soubirious in 1858, saying "I am the Immaculate Conception" and it has been confirmed by miracles ever since.
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