Understanding the Biblical Basis of the Trinity

Questions about a good approach to take with someone who says the Trinity isn’t biblical, how to respond to Jehovah’s Witnesses who say Jesus received authority to forgive sins in Luke 4:18–19, and whether God “was looking for friends to collaborate with on running the world.”  

• What’s a good tactical approach to take when in conversation with someone who says the Trinity isn’t biblical but was adopted much later in church history?
• How would you respond to Jehovah’s Witnesses who say Jesus didn’t have authority to forgive sins because he was God, but rather he was given that authority as part of his anointing in Luke 4:18–19?
• How would you respond to the claim that “from the beginning of time, God was looking for friends to collaborate with on running the world”?