I never came from a OSAS belief system, but this would be a big sticking point for me. How according to the Calvinist view can there be any real assurance of salvation? Is anyone so faithful, so free from sin that they can say with absolute certitude that they would never fall away? Even Calvinists sin. I’m sure very many daily commit sins that would be considered very serious. Supposedly the holiness or sinfulness of one’s life is an outward sign of one’s election. But since, for the elect, none of there sins are counted against them, and for the reprobate, all of their sins are counted against them and everything that to them is supposedly a sign of grace (baptism, Lord’s Supper etc.) is empty and worthless, there is no proportion between the outward signs of election and the actual reality. Two people can equally appear to be Christians and equally believe inwardly that they are Christians, yet one falls away and the other does not. One really was a Christian all along; the other was in fact never a Christian at all, and his whole Christian life was a total lie. Or imagine two men equally Christian by both inward and outward appearance who did both persevere until death. Yet one had real grace, another only had fake grace and was never really a Christian.
Calvin acknowledges this problem in his Institutes. He dismisses it as not a big problem, but I don’t think it’s convincing.
I am aware it seems unaccountable to some how faith is attributed to the reprobate, seeing that it is declared by Paul to be one of the fruits of election; and yet the difficulty is easily solved: for though none are enlightened into faith, and truly feel the efficacy of the Gospel, with the exception of those who are fore-ordained to salvation, yet experience shows that the reprobate are sometimes affected in a way so similar to the elect, that even in their own judgment there is no difference between them. Hence it is not strange, that by the Apostle a taste of heavenly gifts, and by Christ himself a temporary faith, is ascribed to them. Not that they truly perceive the power of spiritual grace and the sure light of faith; but the Lord, the better to convict them, and leave them without excuse, instills into their minds such a sense of his goodness as can be felt without the Spirit of adoption. Should it be objected, that believers have no stronger testimony to assure them of their adoption, I answer, that though there is a great resemblance and affinity between the elect of God and those who are impressed for a time with a fading faith, yet the elect alone have that full assurance which is extolled by Paul, and by which they are enabled to cry, Abba, Father. Therefore, as God regenerates the elect only for ever by incorruptible seed, as the seed of life once sown in their hearts never perishes, so he effectually seals in them the grace of his adoption, that it may be sure and steadfast. But in this there is nothing to prevent an inferior operation of the Spirit from taking its course in the reprobate. Meanwhile, believers are taught to examine themselves carefully and humbly, lest carnal security creep in and take the place of assurance of faith. We may add, that the reprobate never have any other than a confused sense of grace, laying hold of the shadow rather than the substance, because the Spirit properly seals the forgiveness of sins in the elect only, applying it by special faith to their use. Still it is correctly said, that the reprobate believe God to be propitious to them, inasmuch as they accept the gift of reconciliation, though confusedly and without due discernment; not that they are partakers of the same faith or regeneration with the children of God; but because, under a covering of hypocrisy, they seem to have a principle of faith in common with them. Nor do I even deny that God illumines their minds to this extent, that they recognize his grace; but that conviction he distinguishes from the peculiar testimony which he gives to his elect in this respect, that the reprobate never attain to the full result or to fruition. When he shows himself propitious to them, it is not as if he had truly rescued them from death, and taken them under his protection. He only gives them a manifestation of his present mercy. In the elect alone he implants the living root of faith, so that they persevere even to the end. Thus we dispose of the objection, that if God truly displays his grace, it must endure for ever. There is nothing inconsistent in this with the fact of his enlightening some with a present sense of grace, which afterwards proves evanescent.
And so on. But like you bring up, this idea is contrary to our experience, to say nothing of Scripture and the perennial teaching of the Church.