Here are some more explainations of the words deilois and apistois.
But the cowardly
Cowardly is
δειλοῖς [deilois] : “timid [ones],”
1 “persons showing fear in a shameful way.”
2 The word is used to describe lack of faith (Mtt.
8:26; Mark
4:40).
During the Tribulation, the
Beast worshipers took his mark, partly out of fear of losing their lives (Rev.
13:15+). They were among those who, because of fear, saved their lives, but ultimately lost both their soul and body in hell (Mtt.
10:28). The wicked and lazy servant hid his talent in the ground out of fear rather than investing it for the Lord’s benefit. He was cast into outer darkness (Mtt.
25:25; Luke
19:21). Those who deny Jesus before men, possibly out of fear, will not be confessed by the Son of Man before God and His angels (Luke
12:8-9). Many of the religious rulers believed in Jesus, but for fear of being put out of the synagogue by the Pharisees, would not confess Him.
They loved the praise of men more than the praise of God (John
12:43).
This passage concerns those who are among the
unredeemed, it is not a threat that believers who struggle with fear at times are in danger of the Lake of Fire. Those who are born-again have not been given a spirit of fear, “but of power and of love and of a sound mind” (2Ti.
1:7).
unbelieving
Unbelieving is
ἀπίστοις [apistois] :
ones without faith. Those who did not believe the truth are condemned (2Th.
2:12). Those who are unbelieving have a defiled mind and conscience. They often profess to know God, but by their works they deny Him (Tit.
1:15-16). Lack of faith in the Son results in eternal condemnation:
He who believes in the Son has everlasting life; and he who does not believe the Son shall not see life, but the wrath of God abides on him. (John
3:36)
He who believes in the Son of God has the witness in himself; he who does not believe God has made Him a liar, because he has not believed the testimony that God has given of His Son. (1Jn.
5:10)
There are no unbelieving among the overcomers: “And this is the victory which has overcome the world—our faith. Who is he who overcomes the world, but he who believes that Jesus is the Son of God?” (1Jn.
5:4b-5).
http://www.spiritandtruth.org/teaching/Book_of_Revelation/commentary/htm/032108.htm#3.21.8