A first cause might be the God of Christianity. Being agnostic doesn't rule it out, it says we don't know.
I think many Christians would agree that evil and God's goodness seem incompatible, and this is a real problem. But Christians also believe the Bible where it says God knows better than we do. Really believing this means believing that God knows the reason for things even if we don't know or can't know. Perhaps as JWs and former JWs we have a particular problem understanding this distinction because JWs believe they have a definite answer to the problem of evil. Many other Christians will candidly admit they don't have a solution, but they trust God.
I am reminded of those legal cases you sometimes hear about where the evidence against someone seems overwhelming, perhaps even including DNA or fingerprint evidence, and almost everyone believes they are guilty. But they protest their innocence and one family member or partner sticks with them for years despite all the evidence. Eventually some new unexpected evidence emerges to reverse the guilty verdict. Sometimes they may ask the partner how they knew. They may simply say: "I just believed him". It's an intuition, a trust, a conviction, not based on identifiable evidence, but not irrational or stupid either.