There’s so much to be set on this subject. One might wonder, what Abraham, Isaac and Jacob thought. Granted, the Lord God did tell them particular things about their seed; but how would’ve they thought if God had not imported this truth to them.
I’ve struggled over the years pondering this thinking. Much can be said for having the faith of a mustard seed. But, having faith in the promises of God, to our seed and those that are afar off, should all boil down to hope, especially in light of Ishmael and Esau. As well, I believe we can take for granted, that the names mentioned had a sound theology, knowing that the Covenant has internal and external distinctions.
Parents are commanded to rear their children and the way they should go, acknowledging these distinctions.
I don’t believe that it shows a lack of faith, understanding these truths, and moving forward in such truth. It’s not like, God is going to unelect an individual for a lack of faith in this respect; or that, he is going to change his decree based on a positive faith toward the child.
Consider the prayer of Abraham when he asked God, ‘oh that Ishmael might live before thee’. This was a prayer that was as sincere as anybody could ever have prayed.
In regards to the comment above, asking, whether or not the thinking has changed over the ages, I would say it has in many ways. It would seem that some of the Divines and some of the church antiquarians held to more of a spiritual treatment of this subject- especially what baptism actually means.
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