This is a huge subject, worthy of much more than I can say at this time. However, let me add a few comments.
In John 14:6 Jesus said: “I am the way and the truth and the life. NO ONE comes to the Father except through me". This means no one can ever work hard enough themselves to please the Triune God of Christianity.
On our own God sees us as unrighteous however many good works we do. Our good works are seen as " filthy rags" by Almighty God (Isaiah 64:6).
This is why our Heavenly Father sent His Son to earth to pay the ransom for our sins (filthiness) by dying on the cross. Christianity is the only religion which recognises that we can NOT save ourselves. Christians are not arrogant in believing their human ‘fallen’ selves, could ever be seen as righteous before Almighty God. It is only IN Jesus Christ that God can look on us and accept us as righteous. All other religions believe if they do enough good works they will be saved. The New Testament unequivocally teaches this is an error.
Everything changed when Jesus Christ came to the earth for the first time. If we believe in Him, and repent of our sin God sees us as righteous IN Christ, and God will accept us into His Holy Kingdom when we leave this planet.
The Trinity/Triune nature of God is essentially a Mystery which includes: God The Father, The Son (Jesus Christ) and The Holy Spirit. Jesus Christ sent us The Holy Spirit when He returned to Heaven on Ascension Day. The Holy Spirit is the third equal part of the Godhead, and He is a person, it is not something ethereal and wispy. The Holy Spirit is the One who leads us, befriends us, and counsels us, whilst we live on earth.
Jesus Christ did not come to destroy the Law of the Old Testament, NO HE FULFILLED it (Matthew 5:17).There are so many phrophecies about the arrival and mission of Jesus Christ throughout the Old Testament; He is everywhere in this book.
With Jesus Christ we are sure of our Salvation; in the complete Bible this is an unequivocal teaching. As to what God does with all the other believers and non-believers, the Bible does not make clear; it uses so many different metaphors which can be misinterpreted and difficult to understand fully.
One thing I do know is that God Almighty is primarily a loving and forgiving God, but there will be consequences for those who do not seek God through Jesus’s Strait Gate which leads to The Narrow (safe) Way. Going via the alternative broad and impossible to navigate way to God, results inevitably in lostness (see Matthew 7:14 ). Jesus Christ is the only One who leads us in the sure Way, His Way is the only secure Path Home.
As for the others who also seek God: they will not be in the same place as Christians because Jesus Christ tells us that He goes to prepare a place for those who believe in Him (John:14:1-3).
I do not believe a loving God burns people in eternal hellfire because they turn away from Him and/or get ‘lost’. Instead God’s Fire is a consuming fire like the one which Moses saw in the burning bush (Exodus 3:1-); this Fire cleanses, repairs and heals.
For me, hell would be separation from The Triune God I have come to love and know intimately; this knowing Him is so much more than just knowing about Him. I could not bear a life without Him by my side.