The Scripture teaches us that anyone who truly believes the gospel message is saved. This is true regardless of their religious affiliation. The question for anyone is really, “What does a person actually believe?” The belief or faith the Bible speaks of is a faith alone in Christ alone apart from religious works (Eph. 2:8-9; Tit. 3:4-5). Salvation is a free gift (Rev. 21:6; 22:17).
Many people believe in the historical facts about Jesus Christ—His incarnation, perfect life, death, and resurrection, but they add to this a system of works as part of what is needed for salvation. In other words, they add to the gospel message. It is not simply faith alone in Christ alone, but faith plus some work. This is precisely what the religious Jews were doing in the time of Christ and the apostles. They were seeking to add religious works such as circumcision. Paul called this a gospel of another kind which, in essence, is not the gospel. The books of Galatians and Colossians were written to refute and show the heresy of this. See Romans 10:1-4.
To believe in Christ is to turn from one’s sources of trust (good works, personal morality, etc.) and to place one’s trust in the person and work of Christ as the sole basis of eternal life with forgiveness and acceptance as a child of God.