Even a purely moral act that has no hope of any immediate and visible political effect can gradually and indirectly, over time, gain in political significance.

This quote means that even if an action has no immediately seen political effects, it can still have a political significance over time. It may be a seemingly small or moral action, not done with any political intentions, but eventually, it can make a difference in political terms. For instance, people volunteering in their local community can help to create a sense of togetherness, which can lead to people supporting similar political causes together. This kind of indirect effect can take some time but can still be important in the long run.