In general, I think a lot of Christians — and perhaps not just Protestants — believe that Sunday school is the way for their children to learn about God. However, when we examine the Scriptures that is false.
Deueteronomy 6:1 “Now this is the commandment, the statutes and the judgments which the Lord your God has commanded me to teach you, that you might do them in the land where you are going over to possess it, 2 so that you and your son and your grandson might fear the Lord your God, to keep all His statutes and His commandments which I command you, all the days of your life, and that your days may be prolonged. 3 O Israel, you should listen and [a]be careful to do it, that it may be well with you and that you may multiply greatly, just as the Lord, the God of your fathers, has promised you, in a land flowing with milk and honey.
4 “Hear, O Israel! The Lord is our God, the Lord is one! 5 You shall love the Lord your God with all your heart and with all your soul and with all your might. 6 These words, which I am commanding you today, shall be on your heart. 7 You shall teach them diligently to your sons and shall talk of them when you sit in your house and when you walk by the way and when you lie down and when you rise up. 8 You shall bind them as a sign on your hand and they shall be as [b]frontals [c]on your forehead. 9 You shall write them on the doorposts of your house and on your gates.
Spiritual education was and is supposed to be to continuous and daily walk. It is supposed to be from fathers to sons and the entire household. Church — or rather the gathering of believers — serves to reinforce the Christian values that are taught by fathers to sons.
With that said there’s no excuse for your average Christian to be uneducated about what Jesus and the Scriptures speak. We live in a time where you have access to hundreds of different translations of the Bible at your fingertips. We have compilations of writings of the early Church fathers.
We are perhaps the most spiritually rich in resources of any generation in history, yet we are headed toward being one of the most spiritually bankrupt generations.