What is the downside of believing that?
A whole lot is wrong with this. It is being argued that because an infant, child, or unbelieving spouse, is in the home of a believing adult that unbeliever is saved. Scripture is very clear.
John 14:6 Jesus said to him, "I am the way, the truth, and the life. No one comes to the Father except through Me."
If Reformers want to create new "traditions" just like the RC's have done don't get upset when they claim beliefs are legitimate that are not in Scripture. Scripture is silent on this issue. It's an area where we should just have faith, not create beliefs that sprinkling water on a baby's head and saying because their parents are believers that the baby is going to heaven.
WOW, d00d! There it is, in a nutshell! That which we KNOW is plainly declared in the Scriptures. That which we think we know... not so much.
Folks can often forget that at some level, the Bible is written for the common man (milk and meat)... and that it is built as a building would be: From the foundations upward, just like the Prophets are...
Don't get me wrong, meat is better than milk - But if the "meat" you believe in contradicts those common themes which make the milk, Or if the New Testament is believed to contradict the Old, something is wrong with the interpretation... and one must look deeply into the nearest mirror to find the fault.