God had an angel (Satan) turn against Him and take a third of the angels with him. What prevents this from happening again after Satan is destroyed in the Lake of Fire?
A survey of scripture teaches that demons (i.e., fallen angels) have become corrupt in nature like fallen man. These will not be redeemed since Hebrews 2:16 teaches that the Lord does not give help (i.e., a way of redemption) to fallen angels. We also know that one-third of the angelic host were convinced by Satan to rebel against God with him, according to Revelation, meaning that the remaining two-thirds of the angelic realm did not follow Satan.
Unlike mankind, all angels were created at once rather than through a process of procreation. We know this because Jesus said in Matthew 22:30 that angels are not given into marriage, which implies they do not procreate. We must conclude that when Satan fell, he had the entire host of angels at his disposal, and of these, he convinced a third to follow him into rebellion. Notably, two-thirds of the angels were not fooled by Satan and remained faithful.
Therefore, we can safely assume that the remaining angels will never fall, since the author of lies failed to draw them in his first attempt and there will be no future source of rebellion in God’s creation according to Revelation 21. Simply put, if Satan couldn't convince the remaining angels to rebel in his earlier attempt, then they will remain faithful forever.
Likewise, we can assume that once we obtain our glorified state in the Kingdom, we will also be impervious to Satan’s schemes. There will be no second fall of man, because the source of the fall (i.e., Satan and his lies) has already been judged, according to scripture. Scripture says we are no longer slaves to Satan and have conquered him through our faith in Jesus Christ. We have been made new in Christ’s likeness so that we share in Christ’s nature and in his desire to obey the Father.
Therefore, both the faithful angels and believers in Jesus Christ (i.e., the elect angels and elect humanity) are saved by God and forever preserved from rebellion. God’s original plan for Creation allowed for and even desired a fall of both the angels and of mankind. Self evidently, the Lord permitted His creation to be susceptible to deception and rebellion in the first place, so that a fall would occur in both realms.
Ultimately, the Lord brought about these circumstances so that He might redeem the elect and thereby bring glory to His name for His mercy and love. This is the essence of Paul’s exclamation in Romans 8:
Rom. 8:31 What then shall we say to these things? If God is for us, who is against us?
Rom. 8:32 He who did not spare His own Son, but delivered Him over for us all, how will He not also with Him freely give us all things?
Rom. 8:33 Who will bring a charge against God’s elect? God is the one who justifies;
Rom. 8:34 who is the one who condemns? Christ Jesus is He who died, yes, rather who was raised, who is at the right hand of God, who also intercedes for us.
Rom. 8:35 Who will separate us from the love of Christ? Will tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword?
Rom. 8:36 Just as it is written, “For Your sake we are being put to death all day long; We were considered as sheep to be slaughtered.”
Rom. 8:37 But in all these things we overwhelmingly conquer through Him who loved us.
Rom. 8:38 For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers,
Rom. 8:39 nor height, nor depth, nor any other created thing, will be able to separate us from the love of God, which is in Christ Jesus our Lord.
You might find our Romans study helpful concerning this question.