Yes, He is, and by that, and that He being the one who brough salvation to fruition, it is His right, and prerogative, to bestow it upon
whomsoever He has chosen for it, with no disagreement nor modification to it possible from/by us. He is God.
[Rom 9:14-15, 18, 21 KJV]
14 What shall we say then? [Is there] unrighteousness with God? God forbid.
15 For he saith to Moses, I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion. ...
18 Therefore hath he mercy on whom he will [have mercy], and whom he will he hardeneth. ...
21 Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?