He will bring forth your righteousness as the light, and your justice as the noonday. -- Psalm 37:5-6 (ESV)
God clearly does not tolerate idols (Exodus 20:3-4).
“Those who regard worthless idols forsake their own Mercy.
But I will sacrifice to you with the voice of thanksgiving; I will pay what I have vowed.
Salvation is of the Lord.”
-- Jonah 2:8-9
Therefore, you need to choose if it is worth putting your trust in things that fail, like:
- idols (Exodus 20:3-5, Ezekiel 18:6)
- important people (Psalm 146:3)
- wicked people (Psalm 1:1)
- pleasure (Ecclesiastes 2:1-11, Proverbs 21:17)
- money (Matthew 6:24)
Choose this day whom you will serve. -- Joshua 24:15
Whoever would draw near to God must believe that He exists... -- Hebrews 11:6 (ESV)
Therefore, it is not possible to be saved if you do not at least believe that God exist. This already rules out Atheism, Universalism, and many other religions. However, unlike the Gnostic reasoning, to only know that God exist is not enough to save you.
A "good God" is not a god who allow sin. The Lord God is just and will not bend his own rules.
Yet you say, ‘The way of the Lord is not just.’
Hear now, O house of Israel: Is my way not just? Is it not your ways that are not just?
- When a righteous person turns away from his righteousness and does injustice, he shall die for it; for the injustice that he has done he shall die.
- Again, when a wicked person turns away from the wickedness he has committed and does what is just and right, he shall save his life. Because he considered and turned away from all the transgressions that he had committed, he shall surely live; he shall not die.
Yet the house of Israel says, ‘The way of the Lord is not just.’
O house of Israel, are my ways not just? Is it not your ways that are not just?
"Therefore I will judge you, O house of Israel, every one according to his ways", declares the Lord GOD.
Repent and turn from all your transgressions, lest iniquity be your ruin. Cast away from you all the transgressions that you have committed, and make yourselves a new heart and a new spirit! Why will you die, O house of Israel? For I have no pleasure in the death of anyone, declares the Lord GOD; so turn, and live.
-- Ezekiel 18:29-32 (ESV)
For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life. For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name [authority] of the only Son of God. -- John 3:16-18 (ESV)
Who is it that overcomes the world except the one who believes that Jesus is the Son of God? -- 1 John 5:5 (ESV)
Jesus is the Christ (Messiah)
Now Jesus did many other signs in the presence of the disciples, which are not written in this book [John]; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name. -- John 20:30-31 (ESV)
Who is the liar but he who denies that Jesus is the Christ? This is the antichrist, he who denies the Father and the Son. No one who denies the Son has the Father.
Whoever confesses the Son has the Father also. -- 1 John 2:22-23 (ESV)
That if you confess with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. -- Romans 10:9
God longs “that every tongue should confess that Jesus Christ is Lord” - Philippians 2:11
Jesus said: "I am the way, and the truth, and the life. No one comes to the Father except through me." -- John 14:6 (ESV)
This is also how the first believers in the New Testament understood it:
There is one God, and one Mediator between God and men the man Christ Jesus, who gave himself a ransom for all, to be testified in due time. -- 1 Timothy 2:5 (NKJV)
Jesus also warned:
Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. Because narrow is the gate and difficult is the way which leads to life, and there are few who find it. -- Matthew 7:13-14 (NKJV)
There is salvation in no one else, for there is no other name under heaven given among men by which we must be saved. -- Acts 4:12 (ESV)
Laws cannot save:
If righteousness comes through the law, then Christ died in vain. -- Galatians 2:21 (NKJV)
Good works cannot save:
For it is by grace you have been saved, through faith – and this not from yourselves, it is the gift of God – not by works, so that no one can boast -- Ephesians 2:8-9 (NIV)
Obedience cannot save:
No one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin. We all fall short of the glory of God. -- Romans 3:23 (NIV)
If you confess with your mouth Jesus as Lord, and believe in your heart that God raised him from the dead, you will be saved. -- Romans 10:9
Jesus' mission would have been a failure if God did not raise him from the dead. Paul witness:
Now I would remind you, brothers, of the gospel I preached to you, which you received, hin which you stand, and by which you are being saved, if you hold fast to the word I preached to you — unless you believed in vain.
For I delivered to you as of first importance what I also received:
- that Christ died for our sins in accordance with the Scriptures,
- that he was buried,
- that he was raised on the third day in accordance with the Scriptures, and
- that he appeared to Cephas, then rto the twelve. Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. Then he appeared to James, then to all the apostles. Last of all, as to one untimely born, he appeared also to me.
-- 1 Corinthians 15:1-8
Jesus then said to them, “Truly, truly, I say to you, it was not Moses who gave you the bread from heaven, but my Father gives you the true bread from heaven. For the bread of God is he who comes down from heaven and gives life to the world.”
They said to him, “Sir, give us this bread always.”
Jesus said to them, “I am the bread of life; whoever comes to me shall not hunger, and whoever believes in me shall never thirst.”
Jesus answered them, “Do not grumble among yourselves. No one can come to me unless the Father Who sent me draws him. And I will raise him up on the last day.
It is written in the Prophets, ‘And they will all be taught by God.’
Everyone who has heard and learned from the Father comes to me — not that anyone has seen the Father except he who is from God; he has seen the Father. Truly, truly, I say to you, whoever believes has eternal life: I am the bread of life. Your fathers ate the manna in the wilderness, and they died. This is the bread that comes down from heaven, so that one may eat of it and not die. I am the living bread that came down from heaven. If anyone eats of this bread, he will live forever. And the bread that I will give for the life of the world is my flesh."
The Jews then disputed among themselves, saying, “How can this man give us his flesh to eat?”
So Jesus said to them, "Truly, truly, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever feeds on my flesh and drinks my blood has eternal life, and I will raise him up on the last day."
-- John 6:47-54 (ESV)
Unfortunately I cannot quote the entire book of John here, but in short Jesus states that believers should feed themselves with "Jesus".
Obviously Jesus was not encouraging cannibalism, but commonly the majority of scholar will explain that Jesus meant that people should feed on the teaching of Jesus as one needs to feed yourself on food like bread. However, Paul explains that it means "participation in the body of Christ":
The cup of blessing that we bless, is it not a participation in the blood of Christ?
The bread that we break, is it not a participation in the body of Christ?
Because there is one bread, we who are many are one body, for we all partake of the one bread.
-- 1 Corinthians 10:16-17 (ESV)
This means that believers should not live in isolation, but that believers need each other and should support each other as one need to eat food every day to stay alive.
In other words, salvation is not a once off "sinner's prayer", but you need to daily feed yourself daily with the "bread of life" or you will simply "have no life in you".
In a practical sense this could include anything from praying, teaching and encouraging each other, to rendering skilled services or providing resources like physical food, clothes, money, shelter, etc.