A lot of people, Christian and non-Christian alike, think of holiness as not doing certain things: don’t drink or cuss or chew or run with folks that do. That’s not a biblical definition. It’s certainly not what Isaiah thought about when he saw God.
God is holy. That means at least three different things. He is unique, entirely unequaled in all he created. He is pure and incorruptible. He is separate from sin and from sinners. Yet at the same time, he desires the companionship of his creation, including the sinful human race.
According to the law, a leper had to be expelled from the community. The presence of a leper made the whole community unclean. Think about it. The law also reminded every one of them that they were sinners, but the presence of a leper would make them even more unclean.
Jesus was different. He did not shun lepers. By his grace, he touched them and they were healed. Isaiah lamented his unclean lips, but an angel touched him and purged his sin.
So a holy God does not shrink back from touching sinners. Leprosy and sin flee at his touch. Christians, filled with the Holy Spirit, should not define holiness simply by the particular sins they do not commit. We should live out a holiness that, far from being corrupted by the presence of sin, brings cleansing and healing to whatever part of this unclean world we can influence.