We need assurance that we are God's children, since we may question this when we face suffering. God's Spirit bears witness with our Spirit that we are his children and so are his heirs, sharing in the inheritance of Christ. This means that as we suffer in this life, we suffer with Christ - trusting in him, united to him who suffered for us, that we may be glorified with him. So the very thing that can cause us to question God's love for us, is what he uses to prepare us to be glorified with Jesus! Romans 8:16-17.