“But fornication, and all uncleanness, or covetousness, let it not be once named among you, as becometh saints;”
Ephesians 5:3

In the main verse, he tells us what is becoming of a saint and he explains in subsequent verses that doing things unbecoming of saints; which is the description of the life the unbeliever lives, have excluded them from any inheritance in God’s kingdom:
“For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God. Let no man deceive you with vain words: for because of these things cometh the wrath of God upon the children of disobedience.” Ephesians 5:5-6
He says let no man deceive you, because there are those with words- albeit vain- that could deceive, but as a saint, you have a standard given you by the word of God; our glorious example- Jesus, and he was conscious that he was not of this world and his life was an expression of the consciousness:
“For even hereunto were ye called: because Christ also suffered for us, leaving us an example, that ye should follow his steps: Who did no sin, neither was guile found in his mouth:” 1 Peter 2:21-22