What strikes you as the most important influences on Marx’s and Engel’s understanding of the world before 1845?

Before turning to socialism and materialism in the mid-1840s, Marx and Engels were Young Hegelians, part of the left-wing of the Hegelian philosophical movement. In their joint work, German Ideology (written in 1845/1846), they distanced themselves from their fellow Left Hegelians, especially Ruge and Hess. The Essence of Christianity (1841) by Ludwig Feuerbach won Marx over to materialism and impressed him with its critique of German idealism. Already in his 1841 dissertation on the differences between Epicurus and Democritus, Marx stressed the dynamic, voluntarist side of Epicurean materialism. Marx saw the elements of free choice and self-determination that Epicurus found hidden in the accidental behavior of material atoms as a crucial difference between Epicureanism and the earlier mechanistic atomism of Democritus (see Warren Breckman, Marx, Young Hegelians, and the Origins of Radical Social Theory, Cambridge: Cambridge University Press, 1999, 264).
In his dissertation, Marx (1818–1883) followed Feuerbach in criticizing the individualist idealism of Plutarch, who was one of the principal ancient critics of Epicureanism. He based his philosophical position on the Feuerbachian critique of idealist individualism and combined it with the dynamic materialist atomism that he derived from his interpretation of Epicurean teachings. In 1842, Marx abandoned his dreams of academic employment to enter the field of political journalism; he contributed many articles about political issues to the liberal Rheinische Zeitung. Marx criticized such radical Hegelian theologians as Bruno Bauer for “their detachment from the world” as he sought to ground his ideas in a “concrete analysis of politics and society” (Breckman, 1999, 271).
In 1843, Marx published his Critique of Hegelian Philosophy of Law, which reflected his growing interest in French socialist theorists, notably Saint-Simon and Charles Fourier. Like many other German intellectuals, Marx learned about recent French socialist theories from Lorenz von Stein’s book Socialism and Communism in today’s France (1842). During this period, other Young Hegelians, such as Moses Hess, became deeply interested in forging a synthesis between Hegelian theory and socialism. Breckman argues that in summer of 1843 Marx “made a moral commitment to communism” (p. 283).
Engels (1820-1895) was the son of a prosperous German industrialist. He began reading Hegelian philosophy as a youth, and during his Prussian military service in Berlin he attended the lectures of Hegel’s famous friend and opponent, Schelling, who provided a powerful critique of Hegelian idealism. Like Marx, Engels embraced the Feuerbachian critique of Christianity, and so was disappointed when Schelling turned to religion.
Engels worked in his family business in Manchester, England, beginning in 1842, and he became increasingly interested in the conditions of workers both in Manchester and in Germany. His girlfriend, Mary Burns, was a factory worker in Manchester, and she guided him through the Manchester slums to help him learn about the living conditions of industrial workers. He published his first book, On the Condition of the Working Class in England, in 1845. By this time, he had already become friendly with Marx, whom he first met in 1842. Together, they started to develop their own version of socialist theory along the lines of Marx’s Theses on Feuerbach (1845), in which he argued that Feuerbach’s theoretical materialism lacked a practical transformative relationship to the world. The ideas that Marx expressed in this book and that he and Engels developed further together constituted the beginning of Marxism.