Individuals start out knowing nothing, and then they learn things over time, which constitutes progress in the knowledge of the truth. Of course sometimes they adopt false views, but even false views contain some truth. And sometimes they regress, as by forgetting some truth that they knew, or by abandoning true views and adopting false ones. But surely for most people most of the time, they are learning new things, refining true views, or improving false views so that they are less false. Naturally this will not apply to everyone at all times, but surely it is the norm.
This growth in knowledge will automatically carry over to some degree to communities and institutions, and to humanity as a whole, insofar as each of these is composed of individuals. However, this applies mainly over short periods of time, and there is no such guarantee for communities over longer periods, since individuals die and are replaced by other individuals who need to learn everything over again.
Over time, people have devised ways to overcome this problem to a greater or lesser extent, beginning with tradition. The wise man who has arrived at some knowledge by experience can pass it on to others simply by telling them about it, without the need for everyone to have the same experience. However, this is an imperfect method of transmission, since without experience the thing is not known as well, and sometimes the tradition is distorted, misinterpreted, or ultimately forgotten. Writing was a still stronger means of passing on knowledge, preserving knowledge intact for longer periods of time and allowing a greater possibility for building on the knowledge of the past. The internet is a still stronger means, more or less in the same vein, with a greater guarantee of common knowledge over the entire planet. Thus for example many Eastern European countries under communism used textbooks stating that Galileo was burned at the stake, a historical falsehood. With the internet it becomes easier to eliminate mistakes of this kind.
There are also protocols that apply mainly to certain topics, more convincing ways to transmit truth so that all or the great majority accept it. Thus mathematics and various sciences have become more or less universal, not in the sense that everyone knows them, but in the sense that those who study them basically agree on the conclusions. This allows for the possibility of progress in mathematics, physics, and the like over periods of time much longer than a human lifetime.
There are also many areas which do not have such protocols, such as religion, philosophy, politics, ethics, and the like. This does not mean that there has been no progress in these areas, but it may mean that there has been less. Thus the fact that polytheism is now very uncommon suggests progress in religion.
There are various possible ways that the transmission of truth might be improved in the future, both in general and in regard to specific topics. But even if these ways are not devised, the worst that can happen is that certain areas will stagnate on account of the original problem mentioned, the fact that individuals die and are replaced by others. There is no reason to believe that in these areas considered overall, knowledge will decrease over long periods of time. This means that the overall trend will be one of growth in the truth (since the combination of stagnating areas and growing areas will be an overall trend of growth).
Another way to summarize all of this is that “ways of learning the truth” and “ways of transmitting the truth to future generations” are two technologies, and humanity has been improving both of these technologies, resulting in overall growth in the truth for humanity.