What constitutes sacrilege, and is clapping in church a sin?

No, clapping in church is not a sin; you have the freedom to clap when there is something worth clapping about. For example, last week we clapped for a missionary who spent fifteen years in Africa and came home to tell us his harrowing story; the natural way for a group to say thank you for that kind of service is to clap. Christianity is about that freedom. Remember who wanted to take away the Galatians' freedom: it was the judaizers, insisting on circumcision and dietary laws and the whole burden of the Mosaic law, and Peter said in Acts 15 that we should not lay that yoke on people again. The Church kept the laws that are good for her, the moral commandments, and dropped the burdensome ones, and Paul warns us not to let anyone spy on our freedom in Christ. So you may clap whenever it is fitting. Of course there is inappropriate clapping, and there the pastor is in control of the church; the same goes for other disturbances, like a baby who keeps crying and should be taken to the cry room, or people gossiping before Mass when they should worship, which the pastor should address. But the principle stands: as a Christian you have freedom, and no one should take that away from you.