The Westminster Shorter Catechism
Q47. What is forbidden in the First Commandment?
A. The First Commandment
forbiddeth the denying,[1] or not worshipping and
glorifying
the true God as God,[2] and our God;[3] and the giving of that worship and glory
to any other, which is due to Him alone.[4]
[1] Psalm 14:1; [2] Romans 1:21; [3] Psalm 81:10–11; [4] Romans 1:25.
Three things are forbidden in the First Commandment: (1) To deny God, which is atheism; (2) To fail to worship and glorify the true God as God and our God, which is impiety and rebellion; (3) To glorify or worship any other, which is idolatry and profanity.
The Larger Catechism expounds the answer beautifully (WLC 105): The sins forbidden in the first commandment, are, Atheism,
in denying or not having a God [Ps 14:1; Eph 2:12]; Idolatry, in having or
worshipping more gods than one, or any with or instead of the true God [Jer
2:27–28; 1 Thes 1:9]; the not having and avouching Him for God, and our God [Ps
81:11]; the omission or neglect of any thing due to him, required in this
commandment [Isa 43:22–24]; ignorance [Jer 4:22; Hos 4:1, 6], forgetfulness
[Jer 2:32], misapprehensions [Acts 17:23, 29], false opinions [Isa 40:18],
unworthy and wicked thoughts of Him [Ps 50:21]; bold and curious searching into
His secrets [Deut 29:29]; all profaneness [Tit 1:16; Heb 12:16], hatred of God
[Rom 1:30]; self-love [2 Tim 3:2], self-seeking [Phil 2:21], and all other
inordinate and immoderate setting of our mind, will, or affections upon other
things, and taking them off from Him in whole or in part [1 Jn 2:15–16; 1 Sam
2:29; Col 3:2, 5]; vain credulity [1 Jn 4:1], unbelief [Heb 3:12], heresy [Gal
5:20; Tit 3:10], misbelief [Acts 26:9], distrust [Ps 78:22], despair [Gen
4:13], incorrigibleness [Jer 5:3], and insensibleness under judgments [Isa
42:25], hardness of heart [Rom 2:5], pride [Jer 13:15], presumption [Ps 19:13],
carnal security [Zeph 1:12], tempting of God [Mt 4:7]; using unlawful means
[Rom 3:8], and trusting in unlawful means [Jer 17:5]; carnal delights and joys
[2 Tim 3:4]; corrupt, blind, and indiscreet zeal [Gal 4:17; Jn 16:2; Rom 10:2;
Lk 9:54–55]; lukewarmness [Rev 3:16], and deadness in the things of God [Rev
3:1]; estranging ourselves, and apostatising from God [Ezk 14:5; Isa 1:4–5];
praying, or giving any religious worship, to saints, angels, or any other
creatures [Rom 10:13–14; Hos 4:12; Acts 10:25–26; Rev 19:10; Mt 4:10; Col 2:18;
Rom 1:25]; all compacts and consulting with the devil [Lev 20:6; 1 Sam 28:7,
11; 1 Chr 10:13–14], and hearkening to his suggestions [Acts 5:3]; making men
the lords of our faith and conscience [2 Cor 1:24; Mt 23:9]; slighting and
despising God and His commands [Deut 32:15; 2 Sam 12:9; Prov 13:13]; resisting
and grieving of His Spirit [Acts 7:51; Eph 4:30], discontent and impatience at
His dispensations, charging Him foolishly for the evils He inflicts on us [Ps
73:2–3, 13–15, 22; Job 1:22]; and ascribing the praise of any good we either
are, have, or can do, to fortune [1 Sam 6:7–9], idols [Dan 5:23], ourselves
[Deut 8:17; Dan 4:30], or any other creature [Hab 1:16].