Many Christians claim that they believe in God, but what does that mean?
Some merely believe that Jesus existed as a historical figure. Some want to follow him, but they don't have the time to pay him much attention. Some attend church regularly, but nothing happens much to them spiritually; they continue to "do their own thing" in life. Some pray to God only when they face a crisis or just have a need to fulfill. Some believe sincerely, but when they face a test, they give up. Some only accept the teachings of the Church they like and reject or at least not try to face those they don't feel comfortable with. Some only attend Mass for Christmas and Easter yearly, thinking that maybe this is sufficient to keep God happy. Some believers always have questions about their faith. Then there are those who study and know a lot about Jesus, but don't know him personally. This reminds me of a non-Christian tour guide in Holy Land whose knowledge of Jesus was most impressive. There are others who find one priest or fellow Christian offensive or just unfriendly to them and leave the Church. Some never bother to take the time to study or learn the teachings of the Church. Some think that they are good enough believers, compared to others. Since the word "sin" is rarely heard in homilies nowadays, some have forgotten about the importance of repentance as a precondition for growth in holiness.
The above is not meant to criticize anyone, it's just assessed reality and we can certainly and should do better with prayer and God's help.