Is it ethical to condemn another person's religion? Should the followers of Christ just show love rather than criticize what others believe and practice?
Not everything done in the name of religion is right. Respect for God's word demands that we love what is right and "hate every false way" (Psa. 119:97, 104).
Jesus condemned the Sadducees for denying the resurrection (Matt. 22:23-33). He charged the Pharisees with following their own rules instead of God's, a practice which makes worship useless (Mk. 7:1-13).
Jesus' apostles followed His example of condemning error. Paul commanded the people of Athens to quit worshipping idols (Acts 17:23-31). James warned against vain religion (Jas. 1:26). John wrote against false prophets (1 Jn. 5:1-3). Peter exposed the scoffers who denied that Jesus would come again (2 Pet. 3:1f).
It is not lack of love that leads one to expose error; in fact, the opposite is true. Love for God, love for truth, and love for people's souls motivates Jesus' followers to speak out against whatever is false, whether among the religious or the irreligious. Those who are going the wrong way need to be warned. Of course, we must always speak kindly, "with gentleness correcting those who are in opposition" (2 Timothy 2:25).