Summary: Faith is trusting in God's ability to do what He said He would do. It is the growing gift of God.
A pastor who visited an old man suffering from painful rheumatism found him with his Bible open in front of him. The minister noticed that the word "proved" was written repeatedly in the margin. He turned over a few pages and found, "God is our refuge and strength, a very present help in trouble." "Proved." And so it went on through the Book. Next to John 1:12 (quickview)  he had written "Proved." He had received Christ by believing and had indeed become a child of God. He had proved that promise of God's Word. Millions of other born-again believers could write "proved" next to the key verse of our text this morning (“being fully persuaded that God had power to do what he had promised.” -Romans 4:21 (quickview)  NIV). There isn't a single one who has put this promise of God to the test and been disappointed. (E-Sword Sermon Illustrations)
Chapter 4 of Romans is often used to illustrate the great faith of Abraham and it does do that well. However, in light of verse 21, it becomes clear that this section of the Bible is not so much elevating Abraham’s faith as it is pointing to its source.
In the preceding portion of this chapter in verse 16 Paul, under the inspiration of the Holy Spirit, writes, “Therefore, the promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham's offspring – not only to those who are of the law but also to those who are of the faith of Abraham. He is the father of us all.” (Romans 4:16 (quickview)  NIV)
He is saying that the reason that Abraham was called by God to be a great man of faith was to demonstrate that since it was only His faith which made Abraham a great man, the source of His greatness was not in himself, but in God’s promise.
Faith is not so much an act of our will, though at times faith can describe our response to God. Faith is primarily the instrument of God’s grace. Abraham’s faith was built upon, founded on; it resided in, one reality.
Abraham was “fully persuaded that God had power to do what he had promised.” (v.21) Are you? Are you fully persuaded that God has the power to do what He has promised? Are you – like the gambler with confidence in his hand – “all in?”
In the opening verse of our text today it says “the promise comes by faith.” (v.16) Perhaps we do well to clarify what we mean by faith. In postmodern terms, that is the driving pop cultural and philosophical ideals of our day, faith can mean many things; most often it is a rather nondescript vague sort of ideal.
What does the Bible mean when it speaks of faith? The Holman Bible Dictionary defines faith this way: “Trusting commitment of one person to another, particularly of a person to God. Faith is the central concept of Christianity. One may be called a Christian only if one has faith.”
The Greek word which is translated here as “Faith” is “pistis” and it is found 228 times in the Textus Receptus Greek Manuscript. The reason this is of significance is because it is the prevailing theme of the New Testament.