Feelings of disappointment are not inherently sinful, even disappointment with God. However, we should not stay in our disappointment but move forward in trusting in Him and looking forward to the good things He has for us.
Disappointment is something we all experience this side of eternity. Disappointment with God is also something that believers and unbelievers can experience. Oftentimes, our disappointment has to do with our timing not lining up with God’s timing or God not doing things like we think He should. In such cases, it is good to remember that God doesn’t think like us (Isaiah 55:8–9). Even though we might not always see it immediately, God’s plans are good (Ephesians 2:10), and they will come to pass because He is a God of faithfulness. But they will happen in His timing and in His way. In John 14:1–3, Jesus tells His disciples: “Let not your hearts be troubled. Believe in God; believe also in me. In my Father's house are many rooms. If it were not so, would I have told you that I go to prepare a place for you? And if I go and prepare a place for you, I will come again and will take you to myself, that where I am you may be also.” Similar to the disciples, we must not let our hearts be troubled when we are facing difficult circumstances, such as disappointment with God. Instead, we can look forward to the good things He has in store for us, even if we can’t see them yet, or they are in the next life.
Disappointment with God is a difficult emotion. It can cause us to question God's character, our commitment to God, our worth, and so on. When we are disappointed with God—or, more accurately, with what we perceive God to be doing in our lives—we can find ourselves angry, despondent, or simply resigned. However, we are not without hope. Although in Jeremiah 29:11, God is speaking specifically to Jewish exiles, we can see His character that is still active today: “For I know the plans I have for you, declares the LORD, plans for welfare and not for evil, to give you a future and a hope.” We may not always understand what God is doing in our lives, and it is not wrong to be disappointed with Him. However, as God’s adopted children, we must remember that God is worthy of our trust and commitment. He is the Good Shepherd who will protect His flock—He will not abandon us: “My sheep hear my voice, and I know them, and they follow me. I give them eternal life, and they will never perish, and no one will snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand. I and the Father are one” (John 10:27–30).