Every knee shall bow and every tongue shall confess that Christ is Lord (Philippians 2:9-11) and to God (Romans 14:11), and this confession and belief in him is what leads to salvation (Romans 10:9). Everyone who sees Christ and believes will have eternal life (John 6:40). God loved the world, not merely some of it, and for this Christ was sent to give eternal life (John 3:16). It is not the will of the Father that any child should perish (Matthew 18:14). The good news is for all people (Luke 2:10). All flesh shall see the salvation of God (Luke 3:5-6). The news of the kingdom of God is preached, and every one enters it (Luke 16:16). The Lamb of God takes away the sin of the world (John 1:27). He came not to condemn the world but to save it (John 3:17). All things have been given to Christ (John 3:35), and all who have been given to Christ will come to him and not be cast out (John 6:37), and Christ shall lose nothing that has been given to him but instead will raise us up in the last days (John 6:39). All men will be drawn to Christ (John 12:32). All flesh has been given under Christ’s authority, and all that are given to him shall have eternal life (John 17:1-2). Christ will restore grace in abundance much more than Adam’s sin condemns many (Romans 5:15), and he leads to justification and life to all men just as Adam condemned all men (Romans 5:18). God will have mercy on all (Romans 11:32). For as in Adam all die, so also in Christ shall all be made alive (1 Corinthians 15:22). Christ died for all, and doing so redeemed all (2 Corinthians 5:14-15). All shall be united in fullness with Christ (Ephesians 1:7-10). All shall be reconciled with him (Colossians 1:19-20). Christ is all, in all (Colossians 3:11). God desires all to be saved, and Christ died in ransom for all (1 Timothy 2:3-6). Christ is the savior of all people, especially those who believe (1 Timothy 4:10) but not exclusively. God brings salvation to all people (Titus 2:11). Christ tasted death for everyone (Hebrews 2:9). He propitiates the sins of the whole world (1 John 2:2), the Savior of the world (1 John 4:14), everyone shall give him honor and glory and praise and blessing (Revelation 5:13), he makes all things new (Revelation 21:5).
I believe in it because all of scripture proclaims it loudly. Not one shall be lost to Christ. Not one shall escape God’s love. Not one shall remain in death. To quote St Julian of Norwich: all shall be well, all shall be well, and all manner of thing shall be well.