The Trinity - Christians believe in a triune God. Three persons in One: God the Father, God the Son, and God the Holy Spirit. This is incredibly hard to comprehend, but man is finite whereas God has infinite wisdom and knowledge. Therefore, because humanity's understanding is limited, we cannot comprehend an infinite God. That is an aspect where faith becomes important. I have faith that although I cannot comprehend how one being can also be three, one day I will understand in Heaven.
The Council of Nicea was a council called together by emperor Constantine in order to determine various universal agreements among all Christians. One of the things discussed was the doctrine of the Trinity.