Some in Galatia left the NT gospel (Gal 1:6,7).
"For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek." Rom 1:16.
"Wherefore lay apart all filthiness and superfluity of naughtiness, and receive with meekness the engrafted word, which is able to save your souls." James 2:21
The gospel is the power of God unto salvation. The gospel is that engrafted word that is able to save your soul.
Tts 2:11,12 "For the grace of God that bringeth salvation hath appeared to all men, Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world;"
Grace teaches one how to be saved, that is, how to live soberly, righteously and godly. So there is a connection between the gospel, the engrafted word and grace. So when the Galatians left the gospel they also fell from grace, fell from the engrafted word, fell from the instructions that grace gives one to be saved.
Since grace teaches, it is NOT UNcondtionally given apart from teaching and teaching requires understanding and obedience. So if one does not CONDITIONALLY have knowledge of the gospel he CANNOT be in grace. So those that leave the knowledge of the gospel (going back to the OT as some at Galatia) leave grace.
2 Pet 3:18 "But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ."
If one leaves grace he cannot grow in grace or in the knowledge of Christ.
2 Pet 2:20 "For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein..."
One escapes the pollution/sin of this world by knowledge grace teaches. Again, leaving this knowledge means one leaves grace.
2 Pet 2:1 "Grace and peace be multiplied unto you through the knowledge of God, and of Jesus our Lord,"
Heb 10:26 "For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,"
One leaves the knowledge of truth, he leaves grace entering back into sin.
Show me one example of one who fell from grace but still saved. IF this is possible then grace is not necessary for one to be saved.