It is the covenant with God as expressed through the Torah.
it forced them to answer difficult questions about their relationship with God
Because unlike the Babylonian and Persians the Jews were monotheistic and had a personal relationship with there god. They also believed they were there god's chosen people through a covenant with their god.
God is sacred because of His nature.Perhaps you meant to ask why the relationship with God is so sacred (important) to Jews. The answer is because of the importance of being close to Him (Deuteronomy 10:20) and because of our gratitude to Him.
Jews believe in God the creator and in the Torah which God gave.
Jews pray to God
Yes, same God for also Christians and Jews.
Christians and Jews believe that there is one true God. Christians and Jews pray to the same God.
The Jews. (Though strictly speaking, Jews do not believe that God is Jewish or that God has any religion.)
Yes, religious Jews think about God and follow His teachings and instructions.
Jews pray to God.
The vast majority of the world's population of Jews was not present.