Why this commandment? Why "remember the Sabbath day by keeping it holy?" The Lutheran Symbols give a unique spin to this commandment - it's not about resting, but about sanctifying. And the Word of God is what sanctifies, what makes holy. It's about giving time to the Word so that the Word can do it's faith-delivering and faith-strengthening job.
The sad fact of our fallen condition is that no one can sustain saving faith on their own. Simply can't do it. We don't have the power to keep our faith alive, let alone strong! "Faith comes from hearing." This is not only true at the beginning, but it remains true throughout the Christian's journey. Cut off from the Word of God, a person's faith grows weaker and weaker and finally dies.
This has import, then, for both how we deal with delinquents (AWOL Christians) and how we deal with those who don't know if they believe at all. To the AWOLs who use their own freedom of will to turn away from the Word of God and the Sacraments of Jesus, we are called to speak a Word that they don't want to hear: WATCH IT! You are endangering yourself - body and soul - by cutting yourself off from the Words of Jesus that alone can keep you in repentance and faith. They don't want to hear that. They want to discuss whatever it was that led them to dislike pastor or parish. Utterly irrelevant! It's not a matter of shaping the congregation up to fit the tastes of every last person who walked away. It's calling those who walked away to repent and come back to hearing the Words of Jesus that give and sustain faith. Without those words, faith dies and so hope dies. For it is only faith that holds onto Christ, His forgiveness, His life unending.
And when we are dealing with those who are inquiring into the faith, the temptation is to turn the spotlot on themselves and to ponder: "Do I believe? Really? How do I know?" But we want them to stop looking at themselves and to keep them looking at our Lord and His sure and certain giving. So the best answer to give to a newby wondering about their faith is to say: "Listen to what Jesus says to you!" Listen to His promises. They'll give you faith. His Spirit is at work in them and wants to sustain you.
The third commandment is vital, then, for it is by hearing that faith comes, hearing the Word of God. Time to lose the excuses; time to cease the internal scouring for proof of faith; time to listen up to Jesus and let His Words do the job that His Spirit empowers them to do.