"I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel." (Genesis. 3:15)

"I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel."
(Genesis. 3:15) 
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God said these words to Satan after the fall of our first parents.

Satan had deceived them into sin, thereby disqualifying them for heaven, which was refused to him due to his sin of pride.

His plan was to disqualify the whole human race for heaven by sinking it into sin.

Due to the actual  sin committed by our first parents,  their descendants would be born with original sin, with which one cannot enter heaven.

Due to His foreknowledge about what would happen after the creation of man God had decided from all eternity to amend for the sins of men by becoming man and sacrificing Himself on the cross.

Only a descendant of Adam can amend for his sin.

So God should be born as a descendant of Adam, which means that He should be born of a human mother.

As God cannot sin it would not be worthy of Him to be born of a woman who has even a trace of sin.

So His mother should be a virgin, free from all sins, including original sin.

Any attempt on the part of Satan to tempt her would have no effect on her, as she would be 'full of grace'.

"I will put enmities

between thee and the woman,

and

thy seed and her seed:

she shall crush thy head,

and

thou shalt lie in wait for her heel."

These words of God to Satan mean :

"Both the woman (Mary)  and her Son (Jesus) will be your enemies.

Mary will defeat you fully. (She will be free from all sins.)

You may hang around her to tempt her,  but it will be to no effect."

To be simple,

Jesus' mother would be without  even a trace of sin all her life.

The words of God to Satan serve as the first Biblical proof to Our Lady's immaculate conception.

Angel Gabriel addressed her at Annunciation, "Hail, thou who art full of grace; the Lord is with thee;"

If a container is 'full of' something, it cannot contain even a little of  any other thing.

When Our Lady is full of grace, there cannot be even a trace of sin in her.

As she is full of grace there is no space for sin in her.

This is another Biblical proof for her immaculate conception.

When Our Lady appeared at Lourdes,  Bernadette asked her what her name was.

She replied,  "I am Immaculate Conception".

Our Lady was conceived without original sin.

It is not enough that we celebrate her feast.

We must try our level best to keep ourselves away from sin with the grace of God.

As an enemy of sin, Our Lady is on our side in our fight against it.

Lourdu Selvam.