Baptism (the sign) and thing thing signified are inseparable, and must be held closely together in our thought, so closely in fact that we can attribute the effect of one to the other.
'There is, in every sacrament, a spiritual relation, or sacramental union, between the sign and the thing signified: whence it comes to pass, that the names and effects of the one are attributed to the other.' (The Westminster Confession of Faith 27.2)
This isn't some pseudo-Catholic theology carried over by the writers of the WCF. The NT writers also saw the two as inseparable; consider:
- Whoever believes and is baptized will be saved (Mark 16:16)
- Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit (Acts 2:38)
- In him also you were circumcised with a circumcision made without hands, by putting off the body of the flesh, by the circumcision of Christ, having been buried with him in baptism, in which you were also raised with him through faith in the powerful working of God, who raised him from the dead (Colossians 2:11-12)
- let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water (Hebrews 10:22)
Hebrews 10:22 is particularly helpful to think about:
'That phrase, which does not set up an antithesis between the internal and the external bur rather in a positive "rhetorical parallelism" joins the inner and the outer in "one indivisible reality".' (p. 110, G Wainwright, "Baptism III: Acts, Hebrews, General Epistles, Revelation" in IVP Dictionary of the NT)
In the Reformation there was a great conflict in which there was really three parties. The Roman Catholics who held that external actions were all that mattered, the Anabaptists and other radicals who held that the only the internal ('spiritual') things mattered, and finally the Magisterial Reformers who refused to separate the two. The external actions condemned us if they were not mirrored by internal reality. Our internal faith had to rest on external things (the Word and the Sacraments both of which the Radicals rejected) and could not exist apart from them. Luther saw the need for both better than most (Calvin sometimes almost seemed to view the external things as only accommodations to our weakness and not theoretically necessary):
new spirits assert that faith alone saves, and that works and external things avail nothing, we answer: It is true, indeed, that nothing in us is of any avail but faith, as we shall hear still further. But these blind guides are unwilling to see this, namely, that faith must have something which it believes, that is, of which it takes hold, and upon which it stands and rests. Thus faith clings to the water, and believes that it is Baptism, in which there is pure salvation and life; not through the water (as we have sufficiently stated), but through the fact that it is embodied in the Word and institution of God, and the name of God inheres in it. Now, if I believe this, what else is it than believing in God as in Him who has given and planted His Word into this ordinance, and proposes to us this external thing wherein we may apprehend such a treasure?
But, and this is important, Luther recognised that the order of external and internal things was of huge importance, and had to be got right:
Now when God sends forth his holy gospel he deals with us in a twofold manner, first outwardly, then inwardly. Outwardly he deals with us through the oral word of the gospel and through material signs, that is baptism and the sacrament of the altar. Inwardly he deals with us through the Holy Spirit, faith and other gifts. But whatever their measure or order the outward factors should and must precede. The inward experience follow and is effected by the outward. God has determined to give the inward to no one except through the outward. For he wants to give no one the Spirit or faith outside of the outward Word and sign instituted by him [...]
Observe carefully, my brother, this order, for everything depends on it.
(p.224, Martin Luther, Against the Heavenly Prophets in the Matter of Images and Sacraments (1925), in Selected Writings of Martin Luther ed. T.G. Tappert)