If we can't earn our salvation, how can we lose our salvation by sinning? No one can separate us from God's love; wouldn't that include the devil? After someone puts his faith in Christ, if he was able to lose his salvation it would seem that his actions or thoughts had a part in losing that salvation. If God can't keep us safe from the evil one, that would make Him not all-powerful.
We don’t lose salvation by sinning. We lose salvation by rejecting the sacrifice of Jesus. The road to doing so may be paved with sin, but God can and will forgive sin. “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 continually cleanses us from sin.” (1 John 1:7) When Paul told Galatian Christians that they had fallen from grace it was because they were relying on legalistic adherence to law to save them, not the grace of God.
Nobody except ourselves can separate us from God. Satan may tempt us, but it is our choice to listen to him or not. If the grace of God is a free gift that we can choose to accept, it is also a free gift that we can choose to give back or reject. Does this mean that God is not all-powerful? In the sense that God has limited his own power by giving us free will it does. The alternative is that we do not have free will, but must do exactly what God determines for us. That leads to one of two conclusions. One is that God will save everybody, since sin is an illusion (because it cannot be sin if God made us do it). The other is that God is not love, but arbitrarily chooses to punish some people for doing what he made them do in the first place. Either of those choices goes against what God has taught us about himself in the Bible.
Yes, if one is able to lose his salvation then “his actions and thoughts had a part in losing that salvation.” His actions and thoughts had a part in losing salvation in the first place, when each individual first sins. His actions and thoughts have a part in gaining salvation when he chooses to obey the Lord. So it just makes sense that his actions and thoughts would have a part in the unlikely chance that he would again lose salvation. All that assumes that you accept the concept of free will and reject Calvin’s concepts of Limited Atonement and Irresistible Grace. Otherwise this discussion is irrelevant because God makes me say what I say and makes you believe what you believe, and nothing either of us can say or do will change the other, no matter how hard we try or how hard the other wants to follow God’s truth.