There are many different theories among Christians and even them written about in the bible as to exactly how one obtains the Salvation of God.How do you validate which one is the correct one?
That is a good observation and question.
Since obtain is defined as: obtain: get, acquire, or secure (something).
Then one can't obtain it in the sense it that it is something that they can get, acquire or secure for themselves by any works of their own since it is a gift of God.
1 Tim 2:3-5
3 For this is good and acceptable in the sight of God our Saviour;
4
Who will have all men to be saved, and to come unto the knowledge of the truth.
5 For there is one God, and one mediator between God and men, the man Christ Jesus;
It should be evident that it is not our faith which saves us, seeing that faith is the evidence of the grace in which we hope. So therefore if the substance of salvation is our hope in his grace, then our hope in salvation is no longer hope if it obtained by our faith since faith is the fruit of his grace which is evidence of the fact we obtained it not by our own works but rather it was a gift of God's grace.
Eph 2:8-9
8 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
9 Not of works, lest any man should boast.
Which is to say, if one is saved by their faith, then grace is not longer a gift freely given but a promise which one obtains by their faith; and since men obtain faith not by works but by God's grace, faith would be the evidence seen of the substance of things hoped for, for without faith it is impossible to please God.
Rom 11:29-32
29 For the gifts and calling of God are without repentance.
30 For as ye in times past have not believed God, yet have now obtained mercy through their unbelief:
31 Even so have these also now not believed, that through your mercy they also may obtain mercy.
32
For God hath concluded them all in unbelief, that he might have mercy upon all
Even the Apostle Paul wrote in Galatians the regarding faith being a precept of obtaining grace:
3 For I testify again to every man that is circumcised, that he is a debtor to do the whole law.
4 Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.
5 For we through the Spirit wait for the hope of righteousness by faith.
Gal 5:3-5