Definition of the term 'faith' or what a saving, justifying faith believes in? – John Calvin

“We need to have a clear definition of the term ‘faith’, for without it this teaching will be of no value to us…
When Paul speaks of faith, he does not mean having a vague sense that there is a God reigning in heaven, but of knowing God to be our Father. We can be assured of this because of the promises He has made. We can be joined & united to the Lord Jesus Christ, knowing that all He has is ours & that we have share in it because we are members of His body…

There is, however, a point here which will be of great help to us, & that is to know how Abraham believed in God. Which this, we conclude. If Abraham had simply believed that there was a God in heaven, this could not have justified him, for the pagans believe as much. Or, if Abraham had simply believed that God was the Judge of all the earth, it would not have sufficed. But God said to Abraham, ‘Abram, I am thy shield, & thy exceeding great reward’, and ‘I will be a God unto thee, & to thy seed after thee’, and He also said that all nationts would be blessed in him (Gen. 15:1; 17:7). God spoke thus with Abraham & testified that He counted him a member of His family, one of His own children, & that He would be his God. When Abraham accepted this promise, he was justified. How is this? Well, when God presented Abraham with His bounty & grace, Abraham believed & accepted God’s Word. Thus, his salvation was completely secure.

Now we have a much clearer idea of what it means to be justified by faith. It does not mean we possess a vague notion that God exists, but rather, that we know Him as our Father & our Saviour, since He reveals Himself thus in His Word, & grants us a guarantee of it in the Lord Jesus Christ. Through Him, we are united & joined to God. Although we are wretched creatures, full of wickedness, He will still accept us as His own & find us pleasing in His sight. This is only possible because our Lord Jesus Christ mediates between God and man. Having such a promise, we must rest upon it entirely and not doubt that God will be favorable to us to the end. When we call upon Him, we must find all our refuge in Him, leaving the world behind us and pressing on in the hope of eternal life. This is having faith, & this is being justified! This is how our father Abraham believed. Without this, we cannot be Christians. For until we know what the gospel is really about (as Paul says in Rom. 10:14), we cannot call God our Father. We cannot, he says, call upon a God we have not heard of or believed in. And how can we possibly know Him unless He is revealed to us? Thus, we need faith to go before us. And how do we get faith? By hearing, says Paul (Rom. 10:17). Therefore, we need to be taught the gospel, otherwise we cannot have faith.”

– John Calvin, the true children of Abraham
John Calvin’s sermons on Galatians