Worship

The purpose of going to church is to learn sound biblical doctrine. The Catholic says grace is to be earned, in doing such a thing they have pretty much destroyed the foundation of the Bible.  The Bible says grace is not earned (Ephesians 2:8-9). The Roman Catholic church teaches many different doctrines that aren't in the Bible. These are apostolic succession, worship, and prayer to the saints and Mary, infant baptism, pope/papacy, transubstantiation, plenary indulgences, purgatory and the sacramental system. None of these teachings a solid foundation in the scriptures. These concepts are based on Catholic tradition and contradict the Bible.  

Another reason for attending church is for fellowship. Being devoted to and to honor each other (Romans 12:10), and teach each other (Romans 15:14), to be kind and to be compassionate (Ephesians 4:32), encouragement (1 Thessalonians 5:11), and, love each other (1 John 3:11). This might be possible if there are other born-again believers there. 

The church is a place to take part in the Lords supper. A non-Catholic is not allowed to take part. The reason for communion is to remember the death of Jesus (Matthew 26:28; Luke 22:19; 1 Corinthians 11:26. When taking communion in a Catholic church means that the person agrees and believes in transubstantiation. Most Protestants disagree with the Catholic communion.  

Another purpose is prayer (Acts 2:42). Catholics pray to Mary and the saints. The Catholics believe that doing this is more effective than praying to God directly. This not biblical (Hebrews 4:16). All of us are saints and priests (Romans 1:7; 1 Peter 2:5). Jesus is our High Priest (Hebrews 4:14) and that He is our only mediator (1 Timothy 2:5). 

Why would a true Christian take part in these false teachings? It would be difficult if not impossible to worship God in a Catholic church.  A born-again Christian should not worship in a Catholic church