THE SUNDAY SABBATH KEEPER - WE MUST WARN THEM!
I think that God will discipline those who sin in error but may not destroy them on the day of judgement.
Those who keep Sunday just as we keep Saturday (unto death if necessary) but remain deceived MUST BE WARNED THEY ARE BREAKING GOD'S LAW and wait for God to discipline them.
Those not in rebellion may not be lost but cannot possibly receive the promises made to those who keep His true holy day. Neither can they grow spiritually to become a "young man" because they have not overcome the evil one.
1Jn 2:14 I have written unto you, young men, because ye are strong, and the word of God abideth in you, and ye have overcome the wicked one.
However, there may be those who keep Sunday holy just as we do the 7th day but deep down inside they do it because of fear of man - losing their job, family friends etc. Only God can judge the heart and motives. That is where we cannot judge - they may be lost, they may not be lost.
OUR JOB IS TO WARN THEM.
The blood of the atonement covered all sins of ignorance for Israel (Sins they were not aware of and therefore could not offer a sacrifice) They all would have been lost without the blood atonement - as even a sin of ignorance condemns one to death.
Lev 5:17 And if a soul sin, and commit any of these things which are forbidden to be done by the commandments of the LORD; though he wist it not, yet is he guilty, and shall bear his iniquity.
Under the New Covenant the Blood of Christ (Our high priest) covers sins of ignorance, just as the blood of "Atonement" offered by the High Priest covered Israel.
Heb 9:7 But into the second went the high priest alone once every year, not without blood, which he offered for himself, and for the ERRORS of the people:
1Jn 1:7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
JESUS OUR HIGH PRIEST:
Heb_9:11 But Christ being come an high priest of good things to come, by a greater and more perfect tabernacle, not made with hands, that is to say, not of this building;
Heb_9:25 Nor yet that he should offer himself often, as the high priest entereth into the holy place every year with blood of others;