“I’m not sure the term “unbeliever” can be applied to an infant”
Doesn’t this concern the nature of God and the omniscience of God? His plan encompassed the sin of Adam and its imputation on all mankind. Either the unborn and infants are persons, therefore in Adam, or non-persons until the age of accountability and therefore not in Adam and not born under sin.
If Adam’s sin is not imputed to them at conception, then Job’s complaint that he should not have been born or it would be better if he died before birth is a legitimate complaint for all who never believed. Why go through vicissitudes of life when by death before birth or age of accountability one can be assured of escaping judgment or Obama?
Who is to say that it is not a legitimate complaint?
Secondly, even if Adam's sin is imputed to them at conception, would it not be consistent with the nature of God to impute to infants the righteousness of Christ at death, especially a premature death before such an infant could be tagged with the title of "unbeliever"?
Why go through vicissitudes of life when by death before birth or age of accountability one can be assured of escaping judgment or Obama?
I suspect, unless you are a Mormon, that none of us is capable of making the claim that we asked to be brought into this world, and both our life and our eternal destiny is determined by the grace of God. While there is no specific biblical basis for an "age of accountability" I think it is fairly clear from Romans chapter 1 that God does reveal himself to all men so that they are without excuse, I do not think the same can be said for infants who have no capacity for making any the doctor conclusions regarding the existence of God for their requirement to acknowledge that existence, turn, repent, and follow Christ. Following the logic in Romans chapter 1 all "men" are without excuse, but I do not think the same can be said for infants.