In his decade as China’s top leader, Xi Jinping has asserted greater control for himself and the Communist Party over the country’s economy. Now, Mr. Xi has moved to extend that power more forcefully than ever over China’s financial system. The Communist Party issued a detailed ideological statement on Friday in Qiushi, the party’s main official theoretical journal, that made clear that it expected banks, pension funds, insurers and other financial organizations in China to follow Marxist principles and pay obedience to Mr. Xi. The Qiushi paper, which was being…