This is only for people who believe the Bible to be God's word.
Worshipping in vain means wasting your time because God isn't pleased, and the people are going to hell (Matthew 7:22–23, Matt. 15:13–14; Isaiah 9:16; 2 Peter 2:1–2). Worshipping in vain does exist today and sadly, many don't even know it (Matt. 15:1–9; Mark 7:1–8; Acts 17:23). This is because of man's failure to do things as God has taught in His word. The Bible is here for us to obey and not to add or take from it in any way (Luke 6:26; Matt. 7:21, 1 Corinthians 11:1–2; 1 Peter 4:11; 2 John 8–11; Galatians 1:8–9).
For example, the Bible teaches that Christ is the head of the church, yet men say the Pope is the head (Ephesians 5:23). The Bible teaches that men must worship God in spirit and in truth, yet men will say, "We can worship God as we please," John 4:24. The Bible teaches Jesus built only one true church, yet men will say, "Attend the church of your choice," (Matt. 16:18; Eph. 4:4). The Bible teaches that baptism is essential for salvation, yet men will say that it's alright to baptize babies or that you don't have to be baptized to be saved (Mark 16:16, 1 Peter 3:21, Acts 2:38; Acts 22:16, Gal. 3:27, 1 Cor. 12:13).
Hope to hear from you or see you at the services.