The staple of LCMS preaching was the three - point sermon: you have sinned and by that sin
merited eternal separation from God; Christ died to save you from the consequences of your sins; you should in grateful response strive to live in obedience to God's will.
By even 1 mortal sin — a fully - knowing and fully - deliberate act against God's law (the 10 commandments)-- you reject God, too, and thereby
merit eternal separation from God and punishment in Hell.