natural

It is a law of nature: Living things reproduce after their own kind.
We, as Believers, reproduce by sharing the message, our experiences,
and our faith by our words, our actions--and our very lives. Those
are ways we testify.

One definition of 'testify' is: "Serve as evidence or proof of
something's existence." And, by the way, testifying is not optional.
  

He commanded us to preach to the people and to testify that he is
  the one whom God appointed as judge of the living and the dead. All
  the prophets testify about him that everyone who believes in him
  receives forgiveness of sins through his name." (Acts 10:42-43 NIV)l