To believe something to be true that isn't.
"....Through deceit they refuse to know me, says the Lord."
Jeremiah 9:6 'b'
Deceit can keep us from knowing God, yet falsely believing that we do know Him. We know this is possible from the many "believing" people mentioned in scripture below:
“Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter.Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’ And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness.’ Mathew 7:21-23
Notice a common theme with deceit?
"Through deceit they refuse to know me" and "I never knew you". Deceit gets us to believe we know God when we really don't know Him.
Are you deceived? An easy way to know is to check yourself through scripture:
He who says, “I know Him,” and does not keep His commandments, is a liar (deceives), and the truth is not in him (deceived). -1 John 2:4
But be doers of the word, and not hearers only, deceiving yourselves. -James 1:22
If you keep His commands you are not deceived about your faith, if you don't, then you are deceived. It's that simple. The bible commands us to:
"Examine yourselves as to whether you are in the faith. Test yourselves. Do you not know yourselves, that Jesus Christ is in you?—unless indeed you are disqualified." -2 Corinthians 13:5
If belief was enough, it would not command the reader (who is reading the word because he believes), to examine himself. Genuine belief=Genuine change and obedience to God.
There is a difference between mere intellectual belief and genuine belief:
"Now when He was in Jerusalem at the Passover, during the feast, many believed in His name when they saw the signs which He did. But Jesus did not commit Himself to them, because He knew all men, and had no need that anyone should testify of man, for He knew what was in man." -John 2:23-25
They believed, but did not follow through with obedience, nor had a heart for the Lord. So did they have 'saving belief' in Christ? No. Only intellectual acknowledgement of who He is, but no devotion to Him, no love for Him or faithfulness of following Him. Intellectual faith doesn't save:
"You believe that there is one God. You do well. Even the demons believe—and tremble!"
Even Satan believes Jesus died on a cross and rose again.
Saving faith is submissive to Christ as Lord over our lives, not as a mere title as if just part of His name, "Lord Jesus Christ" but rather, "Jesus Christ, the Lord of my life" that I must obey as Lord.
As it said earlier, “Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter".
God's will for you is to believe on His name as Savior, but then follow through with obedience to His will as Lord, being sanctified (set apart) for Him and not this world:
Jesus answered and said to them, “This is the work of God, that you believe in Him whom He sent.” -John 6:29
For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God; that no one should take advantage of and defraud his brother in this matter, because the Lord is the avenger of all such, as we also forewarned you and testified. For God did not call us to uncleanness, but in holiness. Therefore he who rejects this does not reject man, but God, who has also given us His Holy Spirit. -1 Thessalonians 4:3-8
We cannot do God's will til we are saved, which comes by repenting (turn away/forsake) of sin and trusting (placing faith in) in Jesus Christ by the work He did on the cross to save us. When we come to true repentance and faith, Jesus Christ will change our hearts and renew our minds from corruption of sin and enable us through His spirit to live the genuine Christian life. That doesn't mean we'll be perfect, but our goal will be to become more and more like Him and pursue Him through obeying His word. We must first be transformed (be saved) before we can live like a Christian, just like a caterpillar must be transformed before it can fly like a butterfly:
"And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God." -Romans 12:2
The first step to coming to the truth is to realize you're deceived, so if after you examine yourself (as scripture says) you see that your life is contradictory to the Bible and you are not keeping Christ's commands, then see you have been deceived about "knowing God" and thank God for showing you this so that NOW He can change you. Confess and repent and come to Jesus Christ in real faith. Then, because He has changed you by His Holy Spirit, you will be enabled by His Spirit to live the Christian life through depending on Jesus and abiding in Him:
"Abide in Me, and I in you. As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me. “I am the vine, you are the branches. He who abides in Me, and I in him, bears much fruit; for without Me you can do nothing." -John 15:4,5