Mary (mother of Jesus) was a devout woman who was well aware of God’s scriptures, then why did she doubt Gabriel about virgin birth, when she already knew the prophecy of Isa. 7:14? Did it mean Christians fabricated the prophecy of Isa. 7:14?
Mary was indeed a devout woman. And we can assume that she was familiar with Isaiah chapter seven. You say that she doubted Gabriel’s statement about the virgin birth. Perhaps she did. Can anyone blame her for having some doubts? How would you feel if you were just told that, as a virgin, you would be pregnant? However, there is no record in the Bible of her doubting Gabriel. Who told you that she doubted Gabriel? Where did you hear this? Like I said, perhaps she did doubt some, but if we read the biblical accounts, there is no record of her doubting Gabriel. She did ask a question: “How will this be?” (Luke 1:34). This is not doubt, but a logical/rational question. How will this happen? Then the angel (by the way, you call him Gabriel. He is not named. He is simply “the angel”) explained how it would happen by the Holy Spirit in Luke 1:35-36. Mary’s response was this: “May your word to me be fulfilled.” I see no evidence of doubt here. What I see is amazing faith! I see no evidence of doubt on her part in the Bible. So, the premise of your question is not supported by Scripture.
You ask if Christians may have fabricated the prophecy of Isaiah 7:14. Even if Mary had doubted (there is no evidence of this), the answer is a hard no! The reason is that we have actual manuscripts of Isaiah from the Dead Sea Scrolls from the second century BC! We have manuscripts of Isaiah which predated the birth of Jesus by more than 150 years. Besides, the Septuagint translation from Hebrew to Greek, made in the late 3rd century or the early 2nd century BC also mentions that the virgin will give birth to a son. So, that is a hard no to your question. There is no possible way that Christians “fabricated” the prophecy in Isaiah 7:14. They had no reason to fabricate the prophecy, and they literally could not have fabricated it, even if they had wanted to.