Isaiah 7:​14 Therefore the LORD Himself shall give you a sign; Behold, the young woman is with child, and she will bear a son, and shall call his name Immanu-El. ​ 15 Butter and honey shall he eat, when he shall know how to refuse the evil, and choose the good. ​ 16 For before the child shall know how to refuse the evil, and choose the good, the land whose two kings you dread shall be deserted.

Verse 14 says that God will give Ahaz a sign, this means that Ahaz will see the sign which is the child. Focus on verse 16, the child that Ahaz will see is a sign about 2 enemy kings he is worried about, when he sees the sign he will know that the land of the kings he dreads shall be deserted, this is clearly about something that already happened before Jesus was born. Verse 15 makes it clear that this child was a normal human being who did not always know right from wrong so this can’t be Jesus who Christians say is God and does not sin.