If a person is saved once and for all time why does Satan even bother tempting them to sin? Why would he waste his time on something he can’t change?
Satan tempts believers for several reasons. For example, while he can’t take away our salvation he can steal the joy that accompanies it by enticing us into behavior that embarrasses or shames us and causes us to feel guilty. He can also make a bad example of us to make us unfruitful in our witness to others.
I’ve met and corresponded with a substantial number of believers who live absolutely defeated lives due to the guilt they feel for sins committed during their born again life, and I’m sure there are many more who are living lives of quiet desperation because of the shame and guilt they feel, too embarrassed to even mention it to another person. Satan can’t have their eternal life, but he certainly has messed up their temporal one.
His greatest feat has been to convince them they aren’t worthy of God’s forgiveness, even though Romans 8:1 says there is now no condemnation for those who are in Christ Jesus, and 1 John 1:9 says if we confess our sins God is faithful and just and will forgive our sins and purify us from all unrighteousness.
These are the ones about whom Hebrews 10:27 says, “They have a fearful expectation of judgment that will consume the enemies of God.” It isn’t going to happen to them, but Satan has convinced them this is what they deserve and so this is what they expect.
This is why it’s so important to know who we are in Christ, a new creation (2 Cor. 5:17) with the righteousness of God (Romans 3:2-24) perfect in his sight (Hebr. 10:12-14) with a guaranteed inheritance in eternity (Ephes. 1:13-14). Otherwise we can find ourselves at the enemy’s mercy.