Romans 8:16 The Spirit himself bears witness with our spirit that we are children of God,
According to this verse, true Christians not only believe in God, but know Him. Not all who claim to know God actually do.
James 2:19 “You believe that there is one God. Good! Even the demons believe that—and shudder.”
Matthew 7:21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. 22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ 23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’”
If false prophets are rejected by Jesus, how much more will those who outright deny Him.
Matthew 10:13 “but whoever denies me [Jesus] before men, I also will deny before my Father who is in heaven.”
People who abandon their faith prove they never knew and loved Jesus. According to the evangelical pastors I’ve polled, about 40% of evangelical church members are only Christians in profession, but not in possession.
To explain to others why Jesus is necessary for salvation, get you copy of Why Jesus in the sidebar.