Marx argues that bourgeois ideologists present the prevailing capitalist system as being natural and eternal to make it more difficult to challenge. If something is perfectly natural and operates according to fixed universal laws, then there's simply nothing that can be done to change it. For his part, Marx points out that capitalism, like any other economic system, is a man-made system, which exists to serve the particular interests of a certain social class. As such, it can be changed to make way for a different system that is geared more towards serving the needs of humanity as a whole rather than just the owners of the means of production, the bourgeoisie.
Marx further charges that bourgeois ideologists, in defending capitalism, claim that human nature is unchanging. According to them, humans are fundamentally greedy, selfish individuals, always looking out for themselves. Capitalism is therefore the ideal economic system, the one that most accurately reflects what people are like, and which most effectively satisfies their selfish desires.
But Marx believed instead that human nature wasn't fixed or unchanging; it was largely conditioned by man's social and economic environment. So if people living under capitalism are greedy and selfish, argues Marx, it's because the system encourages them to behave that way. If we want to live in a society in which people come together to work for the common good, then capitalism needs to be replaced by socialism, an economic system based on the principle of "from each according to his ability, to each according to his needs."