When God called Moses to return to Egypt, I find it interesting that Moses used the word Lord
and then effectively said no: "Lord, please! Send anyone else" (Exodus 4:13).
It reminds me of Peter, when he had that vision on the housetop of Simon (see Acts 10:9–16). In his vision, a great sheet was lowered from Heaven with creatures that Peter was forbidden to eat, according to Mosaic Law. God told him, "Get up, Peter; kill and eat them" (verse 13).
But Peter said, "No, Lord" (verse 14).
Those words don't go together. You don't say, "No, Lord." Rather, it is, "Yes, Lord."
Moses was saying to God, "No, Lord, I don't want to do this. I can't do this. I am not the right guy. Send someone else."
This angered God. Moses should have just done what God called him to do.
Here is something to consider. If God is calling you in some way, shape, or form, to not respond actually can be a sin. Yes, God has given you a free will, and I suppose you can live your life as you want to live it for the most part. But if God calls you to do something for Him and you say no, that is disobedience, and that is a sin. To not obey Him is to sin against Him.
"But I am not qualified," you might say. Well really, who is? In fact, often the ones who think they are qualified are disqualified. They're so full of themselves that God won't use them. God likes to use people who don't think they're worthy. God likes to use people who don't think they ever could be used by Him. God likes to use ordinary people in extraordinary ways so that He receives the glory.