100% sincere. (because the Bible says so is not the answer I am looking for. I should have said, why does the Bible say that Abraham believed God?)
If someone ... asks you how you know you are going to heaven when you die (which is the same as how you know all things work together for good), I hope that ... you will not be content to answer, "I know that I am going to heaven because I prayed one time and asked Jesus to come into my heart." Instead, I hope that we will answer something like this: "I know that I am going to heaven because God chose me for his own and predestined me for glory. He has born witness of this in my life by calling me effectually out of rebellion and unbelief and by giving me the declaration of acquittal in his Word. I am justified—my sin went onto Christ, his righteousness went onto me."And now my confidence rests in the covenant oath of God that he will cause me to walk in his will. He who did not spare his Son but gave him up for me, will he not work in me that which is pleasing in his sight (Romans 8:32)? By his Spirit he will cause me to fulfill the just requirement of the law (Romans 8:4), sin will not have dominion over me for I am now under the rule of sovereign grace (Romans 6:14), and that grace will reign through righteousness unto eternal life (Romans 5:21)."The Father planned it ages ago. The Son purchased it centuries ago. The Spirit is causing it today in my heart. It is he who is at work in me to will and to do his good pleasure (Philippians 2:13). And he who began in me this work will complete it at the day of Jesus Christ (Philippians 1:6). It is as sure as the oath and power of the sovereign God. And therefore I know that I am going to heaven, for THOSE WHOM HE JUSTIFIED HE ALSO GLORIFIED."
Better yet, how did God know?