Amandus Polanus (1561-1610) was a German Reformed theologian in the early Post-Reformation era. Below is an excellent exposition of the Reformed position on the sacrament of baptism:
“The faithful that is baptized receives the outward baptism of water, that he may be signified and sealed up unto him, that he is as assuredly washed from his sins, by the blood and Spirit of Christ, as his body is certainly sprinkled & washed with water. Rev. 7:14. Ezech. 36:25.
To be washed with the blood and Spirit of Christ, is to be made partakers of the benefits of the covenant of grace, that is to say, to be reconciled, justified, regenerated, adopted by God, to be his son, to be endued with the freedom of the sons of God, and so forth.
The outward man feels the force of the water: but the inward man feels the powerful working of the blood and Spirit of Christ.
Even infidels are washed with water: but believers only with the blood and the Spirit of Christ.
Therefore not all that are baptize receive remission of sins and regeneration, but the believers only.
To the receiving of baptism, there must be adjoined thanksgiving, which is presently performed by him that is baptized, if he be an adult, or of the years of discretion, or by the witness of his stead, if he be an infant: who yet notwithstanding afterwards when he shall come to years of discretion ought all his life be thankful to God for this benefit.
The peculiar ends of Baptism are.
1. That it may be a seal unto us, of our receiving into the covenant of grace, and fellowship with Christ and the Church, Act. 2:29. Gal. 3:27. 1 Cor 12:13. Therefore Augustine calls baptism, the kingly character or mark, also the character or badge of Christ our Emperor: because that by baptism, as it were by a certain note, Christians are discerned from the other sects, and drawn to acknowledge Christ for their king and Emperor.
2. That by the outward washing, it might represent and confirm unto us the inward cleansing of our souls, which stand in the justification and regeneration, Ephes. 5.26. wherefore baptism does confirm unto every one of us, but that all our sins, original and actual, are forgiven us, for the death of Christ, Act. 2:38. and 22:26. Rom. 6:3. and also that we are clothed with his righteousness, Gal. 3:27. and that we are regenerated by the Holy Ghost, Tit. 3:5. And in the same sense is it said, that baptism saves us, 1 Pet. 3:21. because it seals unto us eternal salvation.
3. To put us in mind of repentance, and of changing our life to the better, Matt. 3:11.
4. That thereby we might be sealed to the certain hope of the resurrection, and of blessed and eternal life.
Now not only those that are of years of discretion, and profess the faith in Christ are to be baptized: but also infants of Christians.
1. Because the very infants are comprehended in the covenant of the grace of God. 1 Cor. 7:14. And therefore both the faith of the parents themselves, and of the Church is confirmed by this sign, that God will be the God and Saviour, as of the faithful parents themselves, so of their seed and children: which promise of his, he at his good time performs in the elect. Rom. 8:29, 30. Tit. 3:5.
2. Because to them also belongs the promise of forgiveness of sins through the blood of Christ.
3. Because they belong to the Church of God.
4. Because they are redeemed by the blood of Christ.
5. Because to them is promised the Holy Spirit.
6. Because they are to be discerned from the children of infidels.
7. Because in the Old Testament infants were to be circumcised.
Therefore every of the faithful one should be but once baptized, as the Israelites were but once circumcised, because we are but once only born. And as circumcision was the nativity or first beginning of Judaism, so baptism is the first beginning of Christianity.
And though we but once baptized, yet is baptism unto us a perpetual sacrament, of the washing from sin, and of our regeneration, that is to say, as baptism does not only evacuate and wash away original sin, but also all other sins, past and present. For they that are baptized, are baptized into Christ’s death. Now Christ’s death is available, not only to wash away those sins, that go before baptism, but those also which in our whole life follow baptism, so that we have not need, to devise a new sacrament, and second table. But does as the Apostle does, who calls the Galatians back to the grace of baptism, Gal. 3:27. So many as are baptized into Christ, have put on Christ.
In baptism original sin is washed away and taken away, specially as concerning the guilt, that is to say, the fault and the punishment, there remaining notwithstanding the vitiation, and the sickness, that is to say, wicked lust and inclination to evil: and that to this end, we might all our life long, fight against sin, and the devil, the author of sin, we in the meanwhile continually calling upon God, and constantly cleaving unto him.”
– Amandus Polanus (1561-1610), The Substance of Christian Religion (Syntagma theologiae christianae), p. 327-332