Can any human with a life span of a few decades personally reach all mankind and serve as a channel of communication from God?

No. But a permanent written record can.
Therefore, would it not be appropriate that the revelation from God be made available in the form of a book?
One of the ancient books claiming inspiration by God is the Bible.
" All Scripture is inspired of God and beneficial for teaching, for reproving, for setting things straight, for disciplining in righteousness,"
states one of it's writers.
{2 Timothy 3:16}

Let us take a closer look at the Bible and see if it is the source of true teachings.