Those who try to separate the teachings of Paul regarding salvation in the Lord Jesus Christ from the teachings of the other apostles are dealt with by Peter in this passage:
2 Pet 3:
14 Therefore, beloved, since you look for these things, be diligent to be found by Him in peace, spotless and blameless,
15 and regard the patience of our Lord as salvation; just as also our beloved brother Paul, according to the wisdom given him, wrote to you,
16 as also in all his letters, speaking in them of these things, in which are some things hard to understand, which the untaught and unstable distort, as they do also the rest of the Scriptures, to their own destruction.
17 You therefore, beloved, knowing this beforehand, be on your guard so that you are not carried away by the error of unprincipled men and fall from your own steadfastness,
18 but grow in the grace and knowledge of our Lord and Savior Jesus Christ. To Him be the glory, both now and to the day of eternity. Amen.
OT salvation was always in the "blood covenant" which God established with man which led to the coming of our Redeemer whose blood covered their sin. NT salvation is always in the "blood covenant" which God established with man which looks back to our Redeemer whose blood covers our sin.
It's as simple and as diffult as that.
Blessings