# Demetrius received a good testimony from everyone

"All who know Demetrius testify that he is a good man" or "Every believer who knows Demetrius speaks well of him"

# Demetrius

He is probably a man whom John wants Gaius and the congregation to welcome when he comes to visit. 

# by the truth itself

"the truth itself speaks well of him." or "everyone who knows the truth knows he is a good person"

# We also testify about him

"We also testify about Demetrius" or "We also speak well of Demetrius"