Romans 3:24 - Being justified freely by His grace through the redemption that is in Christ Jesus. Do you believe the redemption that is in Christ Jesus was FINISHED or UN-finished? Christ's finished work of redemption through His death, burial and resurrection is sufficient and complete to save believers. No supplements needed.
The Bible clearly states in numerous passages of scripture that we are saved through belief/faith "apart from additions or modifications." (John 1:12; 3:15,16,18,36; 6:40,47; 11:25,26; Acts 10:43; 13:39; 16:31; 26:18; Romans 1:16; 3:22-28; 4:5; 5:1; 10:4; 1 Corinthians 1:21; Galatians 2:16; Ephesians 2:8; Philippians 3:9; 1 John 5:13 etc..). *You don't need to add the word "alone" next to "belief/faith" in each of these passages of scripture in order to figure out that the words, "belief/faith" stand alone in connection with receiving eternal life/salvation. Do these many passages of scripture say belief/faith "plus something else?" Plus works? NO. So then it's faith (rightly understood) IN CHRIST ALONE. (Ephesians 2:8,9) Do you believe that we are saved through faith in Christ alone or by faith plus works?
In James 2:19, we see that the demons believe "mental assent" that "there is one God" but they do not believe/have faith in/trust in/reliance in Jesus Christ for salvation. In other words, they do not believe on the Lord Jesus Christ (Acts 16:31) and are not saved. Their trust and reliance is in Satan, as demonstrated by their rebellion in heaven and continuous evil works.