If you preached in an SBC church and did not touch down on the doctrines where you differed, then you compromised - because they Bible COMMANDS you to preach ALL the counsel of God's Word.

As far as that atheist, according to the Bible they were never saved - like the seed that sprung up for a little while but had no root and soon withered away. If someone claimed to be a believer at one time, then later utterly rejects the Lord and His Word, they were never saved to begin with - no matter how much they convinced themselves or others that they were.

1 John 2:19 They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.