Psalm 115: "Not unto us, O LORD, not unto us, but unto thy name give glory, for thy mercy, and for thy truth's sake.
Wherefore should the heathen say, Where is now their God?
But our God is in the heavens: he hath done whatsoever he hath pleased.
Their idols are silver and gold, the work of men's hands.
They have mouths, but they speak not: eyes have they, but they see not:
They have ears, but they hear not: noses have they, but they smell not:
They have hands, but they handle not: feet have they, but they walk not: neither speak they through their throat.
They that make them are like unto them; so is every one that trusteth in them.
O Israel, trust thou in the LORD: he is their help and their shield."
It's easy to imagine that idolatry isn't all that bad. That while we're commanded to worship the true God and no other, we wonder what harm idolatry does in itself.
(A superficial understanding of St. Paul's statement in I Corinthians 8.4-6 -- "As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one. For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,) But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him" -- can also confuse the situation).
But Psalm 115 tells us most succinctly why idolatry is bad: we become like who (or what) we worship. Those who worship false gods who can't see, smell or speak ultimately become unseeing, unable to smell (think the incense of prayer) or speak truly about God or anything else.
Conversely, when we are blessed by the Father to know the true God and worship him, we become like that God.
That's why the creed is so important. Because the creed tells us who God is. And when we tinker with that creed, we're in danger of lapsing into idolatry. And because the first commandment is the source, the fountain, if you will, of all the commandments, worshiping a false god leads us to just about every possible wrong. A correct understanding of God also leads us in holiness, becoming more fully shaped into the image of Christ. Not that we become without sin (the constant returning to God for forgiveness is part of that being shaped in Christ's image) but avoiding gross or subtle idolatry is perhaps the one key to holiness.