God says.
Deuteronomy 12:30 Take heed to thyself that thou be not snared by following them, after that they be destroyed from before thee; and that thou enquire not after their gods, saying, How did these nations serve their gods? even so will I do likewise. 31 Thou shalt not do so unto the LORD thy God:
The Catholics say.
The use of temples, and these dedicated to particular saints, and ornamented on occasions with branches of trees; incense, lamps, and candles; votive offerings on recovery from illness; holy water; asylums; holydays and seasons, use of calendars, processions, blessings on the fields; sacerdotal vestments, the tonsure, the ring in marriage, turning to the East, images at a later date, perhaps the ecclesiastical chant, and the Kyrie Eleison, are all of pagan origin, and sanctified by their adoption into the Church.[Cardinal Newman - Development of Christian Doctrine, pg 373]
Disobedience to God's word.
Ephesians 5:11 And have no fellowship with the unfruitful works of darkness, but rather reprove them.
If you have not personally attended a mass you would not know what it is actually like. I have found that to be similar in nature to the assumptions of some Christians who have no personal experience with Jewish assemblies. Is it a sin for you to attend either Catholic or Jewish assemblies ?
And the TITLE of this thread is...