Marcion says that Moses and The Jewish writings should not be relied upon and does not give a clear view of God The Father whom Jesus Christ revealed to the world.
Christ says, that no one knew Him except The Father (until The Father sent Him of course), and Christ also says that no one knew The Father until He revealed Him:
"All things have been handed over to Me by My Father, and no one knows who the Son is except the Father, and who the Father is except the Son, and anyone to whom the Son wills to reveal Him."
Here is who Jesus Christ said God The Father is. It's different from The Old Testament view:
"...Ye shall be the children of the Highest: for he is kind unto the unthankful and to the evil. Be ye therefore merciful, as your Father also is merciful. Judge not, and ye shall not be judged: condemn not, and ye shall not be condemned: forgive, and ye shall be forgiven: Give, and it shall be given unto you; good measure, pressed down, and shaken together, and running over..."