Fearing and Loving God
Sometimes I hear it said that Old Testament ethics can be summed up in the command to fear God, and New Testament ethics can be summed up in the command to love God.
Related, it is sometimes observed that the Old Testament summary of the law is ten sentences that prohibit ("Thou shalt not" - those making this point usually do so quoting the old fashioned way of putting that), whereas the New Testament command is two sentences that are about love ("Love the Lord your God with all …; love your neighbour as yourself".)