A Jewish friend has asked: If there is only one God, then why must we believe in Jesus? Why can't we just believe in "God" as we always have?

There are two ways to answer this question. First, because the Messiah is the second member of the Triune God, to believe in Jesus is to believe in one God. Second, from the Old Testament onward, God always had a mediator. Under the Mosaic Law, a person could not approach God directly through the sacrificial system but had to approach Him by means of God's chosen mediator, who was the Jewish High Priest. Just as the Old Testament believer needed to go through the High Priest, the New Testament believer must also go through the High Priest. In this case, the High Priest is Yeshua Hamashiach.