For historical figures we have to judge the credibility of the sources. My thought is that we get a gist of the teachings of the master's you mentioned.How can we be sure of what the spiritual teacher like Jesus, Buddha Sakyamuni, Muhammad Actually said out loud in their teachings?
Often in discussion both online and in person to person, one can hear/read they say, Jesus said this or Buddha said this. But how can we verify that it is exactly this that was said? Has the person you speak with actually understood the true words of this great masters from the past? Or is it their own interpretation of what they have read in the different spiritual teachings? and they "forget" to say, this is my understanding of Jesus, Buddha Muhammad's words.
I have fallen for this mistake my self in more than one occasion, but it has made me think, maybe one have to be careful with how we put the teaching forth?
Actually, I learn more from modern spiritual teachers that I respect and revere. Why should we assume that the best stuff for us is only ancient? Can't more modern teachers speak top us more directly and at our level.