Even if we base this question on the base of simple logic – to learn count to ten or to learn to speak properly, write or even have basic societal understanding and orientation you go to school and learn. Logical things are not necessary to be proven thru suttas as they are obvious. So as we can see to learn how to count to 10 you need to highly qualified person to do so, so person who will teach Dhamma obviously need to be at least Sotapanna to see things as they are so he/she can explain it.
A Quest to Recover Buddha's True Teachings