The proof is in the pudding is an old idiom that means, the proof of the pudding is in the eating of it. I think that there is some spiritual truth here as well. This is what I mean. Jesus said that we must be born again to see the kingdom of God. (John 3:3) It's a new life with a new nature and a new appetite. Healthy people have appetites. We get hungry. One of the best signs of a born again believer is a healthy spiritual appetite.
I John 5:
3 For this is the love of God, that we keep His commandments. And His commandments are not burdensome. 4 For whatever is born of God overcomes the world.
I Peter 2:1-3
2 Therefore, laying aside all malice, all deceit, hypocrisy, envy, and all evil speaking, 2 as newborn babes, desire the pure milk of the word, that you may grow [a]thereby, 3 if indeed you have tasted that the Lord is gracious
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