Considering all the statements that God gave us in His Word that have to do with our making the right choices, such as loving Him, following Him, obeying Him, living for Him, seeking the prize, keeping our faith stayed upon Him, being Holy, etc., I would think it is pretty clear that it is definitely our own free choice to be saved or not.
“For many are called but few are chosen.” Matt. 22:14 From the beginning God chose those who would make the choice to love and obey Him and His truth and allow Him to sanctify them through His Holy Spirit. The ones being spoken to in 2 Thessalonians 2:13 are all of the ones who believe in Him and choose to allow Him to live out the same life in and through them that He lived while upon this earth.