... of first importance

Salvation through Jesus Christ isn’t mixable with “salvation” through other religions or self-imposed righteousness. God has given HIS salvation to man the way it is given; through the death and resurrection of Jesus Christ; and it isn’t negotiable.  Salvation has been through the refiner’s fire on Golgotha and nothing can add to it.

1 Cor15 says:

The Resurrection of Christ

15 Now, brothers and sisters, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. ² By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain.

^*3 For what I received I passed on to you as of first importance^[a]:

*that Christ died for our sins according to the Scriptures, ⁴ 

*that he was buried, that he was raised on the third day according to the Scriptures,

⁵ and that he appeared to Cephas,^[b] and then to the Twelve. ⁶ After that, he appeared to more than five hundred of the brothers and sisters at the same time, most of whom are still living, though some have fallen asleep. ⁷ Then he appeared to James, then to all the apostles, ⁸ and last of all he appeared to me also, as to one abnormally born.
⁹ For I am the least of the apostles and do not even deserve to be called an apostle, because I persecuted the church of God. ¹⁰ But by the grace of God I am what I am, and his grace to me was not without effect. No, I worked harder than all of them—yet not I, but the grace of God that was with me. ¹¹ Whether, then, it is I or they, this is what we preach, and this is what you believed.