The main beliefs of a Christian are:-
That God created all things.
That one of the angels (Lucifer) was cast out of Heaven because he rebelled against God. We know him better as Satan.
That God created one man and from that man He created woman. One man and one woman for life.
That man sinned and brought punishment to all who came after him.
That, later, most people on earth were sinning greatly so God decided to wipe out all who lived except one family of 8 who then repopulated the earth.
That. later again, most people on earth were sinning greatly so God sent His Son into the world to show them the way to live for God rather than against Him.
That Jesus performed miracles.
That He taught in the synagogue.
That He was baptised and showed others how to baptise His followers.
That man rejected Him and He was crucified, taking man's punishment for sin so that man would never have to be punished if they confessed their sins and repented. Only Jesus could do this because He is the only one who has walked on this earth as a sinless person.
That Jesus rose from the dead and showed Himself to His friends.
That He ascended into Heaven to be with God but said that He would not leave us alone but would send the Holy Spirit to be a Comforter and Guide.
That might not be 20 but I hope it helps.
In addition to those things there are some things which different churches believe that are not accepted by ALL Christians. Some add their own teachings regarding baptism, the taking of communion, how to make prayer requests, forgiveness etc. so the answers you receive will vary.
-Anonymous