Losing salvation under which Gospel:
Before the cross, the salvation Gospel message was repentance and baptism in water.
After the cross, the Gospel message changed, as revealed by Peter:
Acts 2:36-38
36 Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.
37 Now when they heard [this], they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men [and] brethren, what shall we do?
38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
The great mystery revealed to no other man until Christ revealed it to Paul was/is this:
1 Corinthians 15:1-4
1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
4 And that he was buried, and that he rose again the third day according to the scriptures:
Paul did not preach the Kingdom Gospel where works were required, as preached by James in chapters one and two.
So, under the Kingdom Gospel, one COULD lose salvation:
James 1:22 But be ye doers of the word, and not hearers only, deceiving your own selves.
James 2:14 What [doth it] profit, my brethren, though a man say he hath faith, and have not works? can faith save him?
James 2:17-18
17 Even so faith, if it hath not works, is dead, being alone.
18 Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without thy works, and I will shew thee my faith by my works.
James 2:20-22
20 But wilt thou know, O vain man, that faith without works is dead?
21 Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?
22 Seest thou how faith wrought with his works, and by works was faith made perfect?
James 2:24 Ye see then how that by works a man is justified, and not by faith only.
James 2:26 For as the body without the spirit is dead, so faith without works is dead also.
The Gospel of Grace, on the other hand:
Galatians 3:2-3
2 This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith?
3 Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh?
Romans 3:20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law [is] the knowledge of sin.
Romans 3:27-28
27 Where [is] boasting then? It is excluded. By what law? of works? Nay: but by the law of faith.
28 Therefore we conclude that a man is justified by faith without the deeds of the law.
Romans 4:5 But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.
Titus 3:5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost;
Romans 11:6 And if by grace, then [is it] no more of works: otherwise grace is no more grace. But if [it be] of works, then is it no more grace: otherwise work is no more work.
So, if one has the faith of which Paul preached, originating from the mystery hidden in God from the creation of the world, that man is sealed unto salvation by Holy Spirit, not having to perform works or works of the Law to be counted righteous. Paul spoke the whole Gospel that applies to us today. The claim that the Gospel is the same all throughout, that is a denial of what the four Gospels themselves record, which never included the preaching of Christ crucified and raised on the third day, especially given that the disciples never once preached Christ crucified in those three years they went about preaching. So, no, the claim of the Gospel being the same all throughout is only a denial of the clear reading of scripture for what it says.
MM