These are good follow-up questions. Once we recognise the manner in which baptism saves, that is, as God's pledge to us, then the "effect" of it from the human side includes our pledge to God. This is a covenantal vow of allegiance. So the "answer of a good conscience” is the believer's act of faith responding to God's promise in baptism. It is not the ground of salvation but the appointed way in which the baptised person lays hold of Christ's cleansing. This preserves the objective efficacy of the sacrament, which is God's pledge along with the subjective appropriation of it in our answer. But the priority is given to the objective fact since God speaks first. This avoids turning baptism into either a bare symbol or automatic regeneration.
External membership includes covenant administration, signs, promises, and nurture. Internal membership involves union with Christ, regeneration, faith, forgiveness. All who profess the true religion and their children are covenant members outwardly. Only the elect are members inwardly. The visible church can only act in terms of outward membership, and this is true with respect to adults and infants. Only God acts in terms of inward membership because He knows those who are His and regenerates them where and how He will. As with the tares and the wheat, elect and reprobate are allowed to grow up together to ensure the wheat is not inadvertently pulled up. The means of grace are effectual to the elect among them; hence they must not be denied the means.
All infants of believers will not be saved. Jacob is loved and Esau is hated. Both are children of Isaac. This is used to describe the "present time" of the apostle Paul. There are children of the promise and children of the flesh. Election, not covenant birth, determines salvation. So, no, not all infants of believers will be saved, and yet from the human side we have warrant to hope for the salvation of our children, to pray for them, and to trust that God ordinarily works through the covenant means He has appointed.
[automerge]1770176642[/automerge]
Circumcision was a sign of regeneration as is baptism. But not an empty sign. It is a means of grace in the case of the elect. See my other answers in this thread as to why this is objectively important in the salvation of infants. I have actually given this a lot of thought over the years, as my posts on PB would indicate, as well as the many sermons I have preached where it is relevant in the ordinary course of expository preaching.