Thank you!
1. The fulfillment of
Isaiah 7:14 makes sense, but how did she prove that she was the one referred to, at that time after birth?
2. If God had three parts, why did prior religions not worship him in such a way?
Good questions... and I don't think I have had anyone ask those questions.
It isn't just that one thing that happened to Mary that qualified her (although, to my knowledge, no other virgin had a baby... but rather a multiplicity of reasons:
You have her cousin Elizabeth who also had a child miraculously in her old age that when she say Mary, who was pregnant, was touched by God to say the following: Luke 1:42 And she spake out with a loud voice, and said, Blessed art thou among women, and blessed is the fruit of thy womb. 43 And whence is this to me, that the mother of my Lord should come to me? 44 For, lo, as soon as the voice of thy salutation sounded in mine ears, the babe leaped in my womb for joy.
Then you had the angels of God appearing to the shepherd saying "Luke 2:10 And the angel said unto them, Fear not: for, behold, I bring you good tidings of great joy, which shall be to all people.11 For unto you is born this day in the city of David a Saviour, which is Christ the Lord.12 And this shall be a sign unto you; Ye shall find the babe wrapped in swaddling clothes, lying in a manger. 13 And suddenly there was with the angel a multitude of the heavenly host praising God, and saying,14 Glory to God in the highest, and on earth peace, good will toward men.
You also had the Maggi as well as the sages of the Jewish faith. Matthew 2: 4 And when he had gathered all the chief priests and scribes of the people together, he demanded of them where Christ should be born.5 And they said unto him, In Bethlehem of Judaea: for thus it is written by the prophet, 6 And thou Bethlehem, in the land of Juda, art not the least among the princes of Juda: for out of thee shall come a Governor, that shall rule my people Israel.
John the Baptist confirmed that Jesus was the Messiah as foretold by
Isaiah 40:3. and as mentioned in Luke 3 and
John 1:23
Then you had the prophetess Anna
Luke 2:38 who spoke of Him
I could continue on but the statistical reality of Jesus fulfilling just 8 prophetic statements of the 108 by the Prophets is already in the impossibility realm.
applying-the-science-of-probability-to-the-scriptures
As far as "why didn't previous religions worship that way"... well, I'm not sure they didn't (subtracting those who worshipped false gods.
The Jerusalem Edition of the Zohar, in its comment on Genesis 1:1, explains why the authors believe that God is triune. They point out especially that the name of God, Elohim, is made up of the two Hebrew words El and Haym. El means God and Haym means they; therefore, they conclude that God is a plural being. The Zohar passage is as follows:
Jehovah, Elohaynu, Jehovah. There are three steps to accept the mystery from above. In the beginning God created. In the beginning is the first mystery from where all else spreads…The name takes three forms and from there the name is interpreted in several ways… Rabbi Bechai explained: Elohim is El Haym. These are gods. Remember your creators (Ecclesiastes 12:1). The wise will understand.
Rabbinic Judaism and the Plurality of God
But, regardless, understanding God isn't an easy endeavor. Who can understand all of God? We see Jesus as the fulness of the expression of God.