Man has a free will. We can either obey or disobey God.
Christians can sin willfully. "Willfully" means means
voluntarily, of free will (cf. 1 Pet. 5:2; Ph'm. 14).
Christians have a free will. God has not predetermined who will
be saved and who will be lost taking away man's free will in respect
to sin. Each person must work out his own salvation with fear and
trembling (Phil. 2:12).
Christians are not predestined to salvation. They can fall from
grace (Ga. 5:4; Heb. 12:15).
Abraham illustrates that we have a free will by the fact that God
tested Him (Heb. 11:17; cf. Gen. 22:1). The fact that Christians
are tested connotes that Christians have a free will (Jas. 1:2-3;
1 Pet. 1:7; 4:12; Rev. 2:10; 3:10).
God's command to Jeroboam illustrates the fact that we have a free
will (1 Ki. 11:37-38). It was Jeroboam's decision whether to listen
to all God commanded - to walk in His ways, to do what is right
in His sight by observing His statutes and His commandments.