Satan figured if he could create doubt in opposition to Gods love he would become the authority, ruler of man.
All Satan needed was for Adam to take the bait, in the garden, of his deception in disobeying God.
And you know what Adam did take the bait.
Man became dead spiritually, no longer able to trust that God loved him in a relationship.
But God knew He would send His Son Jesus to earth to establish the opportunity for man to know Him once again.
Because it says in 1st John 3:8
He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
You can rest your faith on this.
Through Christ either one who is unsaved, saved or backslidden or in doubt you can be free.
Jesus died to set you free. He was raised to life for you to call on Him to have victory, for you to call on Him as your Lord in your hour of need.
I thank God through Jesus our Lord. So then with my mind I myself serve the law of God; but with the flesh the law of sin. Romans 7:25
There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit. Romans 8:1