
The opposite of a clean conscience is a defiled conscience which corrupts your ability to be a useful Christian. Here's one commentator on the importance of a clean conscience:
Why is it that we see so many joyless, cheerless, one might almost say useless Christians? Why so many living and walking in such a way as to give the notion of godliness being all gloomy doubt, painful discipline, self-absorbing anxiety, listless musing? Awake! Arise! Shake off the chains that bind you. Go forth and in open day, under the open sky, to meet your God and Father, with your heart open to him, as his heart is open to you. Stand fast in the liberty with which Christ makes you free. Be upright. Be honest, frank, and fearless. Be yourselves; out and out yourselves. Dare to avow yourselves what you are, to God, to your own hearts, to all men. Be of the truth, the whole truth, and nothing but the truth; yourselves true; receiving all truth, declaring all truth; everywhere, and always. Be honest, thoroughly honest, in the closet, in the family, in the market-place, in the parlour. Be transparently honest to your God and Father in heaven. Do but consent to treat him as he treats you. His whole heart, he himself wholly, is yours; all his love; all his fulness. Let your whole heart be his. Be you yourselves his; with no reserve; be altogether, now and for ever, his. (Robert S. Candlish, A Commentary on 1 John [1870; Banner of Truth, 1993], 328)