The [Westminster] Divines confessed that inward images are forbidden: they are the root of the external images that are obviously forbidden. The rejection of internal images is a good and necessary consequence of the rejection of external images. The god we fashion in our hearts and minds are just as idolatrous as icons of the Father, the Son incarnate, and the Spirit.
Q. 109. What sins are forbidden in the second commandment?
A. The sins forbidden in the second commandment are, all devising, counseling, commanding, using, and any wise approving, any religious worship not instituted by God himself; the making any representation of God, of all or of any of the three persons, either inwardly in our mind, or outwardly in any kind of image or likeness of any creature whatsoever; all worshiping of it, or God in it or by it; the making of any representation of feigned deities, and all worship of them, or service belonging to them; all superstitious devices, corrupting the worship of God, adding to it, or taking from it, whether invented and taken up of ourselves, or received by tradition from others, though under the title of antiquity, custom, devotion, good intent, or any other pretense whatsoever; simony; sacrilege; all neglect, contempt, hindering, and opposing the worship and ordinances which God hath appointed (Westminster Larger Catechism, 1647
After the fall we humans are all idolaters at heart. By nature, our first instinct is to fashion gods for ourselves with our hands or in our hearts and minds. When the committee that produced the Westminster Larger Catechism got to the exposition of the Ten Commandments they re-articulated the Reformed rule of worship (i.e., the regulative principle of worship): we may worship God only in the way he has authorized. This rule is biblical and connects us to the ancient Christian church. Other traditions (e.g., the Lutherans, Anglicans, and Evangelicals) ask first whether God has prohibited something in worship. We ask whether God has authorized it. These are distinct views with different outcomes and consequences. Thus, following the consensus of the ancient church, which prohibited images of Christ until the iconodules overthrew that consensus in the eighth century, the Reformed churches across Europe and the British Isles removed images of the holy Trinity, including images of God the Son incarnate, from the Reformed churches and forbid their use.
However much even Reformed people continue to chafe under the rule of worship and the ancient iconoclast position, judging by the objections I have heard from seminary students, fellow ministers, and by the correspondence I have received over the years, one phrase in particular troubles critics and even those who would otherwise subscribe the Larger Catechism: the prohibition of mental images: “either inwardly in our mind…”. Is this phrase warranted or is it an example of over-zealous “Puritans” going just a bit too far in their desire to purify the worship of the churches and prosecute sin to the inner reaches of every man?
The English Reformed gathered at Westminster, who commissioned and comprised the committee did not go too far in Q/A 109. Wilhelmus à Brakel (1635–1711) agreed with the divines:
The seventh sin is to make physical representations of God in our minds. God reveals Himself to the soul of men as a Spirit, doing so in a manner much more devoid of the physical than can be expressed. When the natural man initially thinks upon God, however, he spoils this initial reflection upon God and changes that which is spiritual into something physical. One will either seek to maintain this physical representation of God, finding delight in creating various representations of God in the mind, or it will be contrary to the will of the person engaged in thought, who wishes to have spiritual thoughts of God but cannot do so—this being caused either externally due to people speaking of God, or due to Satan’s influence upon the imagination. The latter is not the sin of the person, but of Satan; that is, if the person is only passively involved, abhorring this, and laboring to resist it (John 4:24).1