God had already sent many prophets through the ages (Moses, et al) to show his people the way. The people did not follow those prophets. They ridiculed most and persecuted many. Then God sent his only Son and they killed him.
It is written: An angel told Joseph in a dream: do not fear to take Mary for your wife, for she is with child of the Holy Spirit, ……He WILL save HIS people from their sins. As Jesus was dying he said to his Father, I HAVE saved all that You gave me, I have lost a one.