If I am going to look at the Scriptures on salvation, I’ll note that the action always begins with God. The famous example I can think of off the bat is Romans 10:14-15:” How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching? 15 And how are they to preach unless they are sent? As it is written, “How beautiful are the feet of those who preach the good news!””
The flow there is that there is an external working upon us – the Word of God is given to us, and then as a reaction there is faith and belief. It starts outside us.
Beyond that, I would contend that the ability to believe is something that is caused and brought about by the Holy Spirit. As a simple example, take Ezekiel 2:1-2 where Ezekiel describes his call by God: “And he said to me, “Son of man, stand on your feet, and I will speak with you.” 2 And as he spoke to me, the Spirit entered into me and set me on my feet, and I heard him speaking to me.”
God speaks (note, there is the Word coming from outside of Ezekiel) and the Spirit acts and causes the response in Ezekiel. The Spirit enters and then the Spirit makes Ezekiel to stand. The source of all of this is God and His action. This is also what is confessed in the Nicene Creed when we speak about the Spirit being the “giver of life” – even life itself is something that is given to us — it is an act of Monergism. God gives – we receive.
This language permeates the Scriptures – it’s a fundamental background throughout. In fact your question itself even supposes it – you ask to prove it without assuming the “impossibility of grace” — but that word “grace” itself denotes a gift, denotes something given by God. You can’t act upon a gift given by God and not have the thrust be God’s working.
I hope this helps – if there was something else you were wanting, send us in a follow up.
Have a good one,
Pastor Eric Brown
Trinity Lutheran Church – Herscher, IL