20 He did not waver at the promise of God through unbelief, but was strengthened in faith, giving glory to God,
Paul brought out the character of Abraham’s faith in this verse.
20 He [Abraham] did not waver at the promise of God through [by] unbelief,
There is a double contrast in Abraham’s faith:
He did not doubt God’s promise
God strengthened his faith
The word “waver” means divided, act as judge, decide between. Sometimes we translate this word as doubt (Ja 1:6). Doubt is a divided mind, a dialoguing mind, a mind that vacillates between faith and uncertainty. Abraham did not doubt God’s promise. He did not waver at the seeming impossibility of having a child by Sarah in his 100th year and her 90th year. He did not permit unbelief to judge God’s promise. The nature of faith is to trust God.
He 10:23, Let us hold fast the confession of our hope without wavering, for he who promised is faithful.
but was strengthened [passive voice—confirmed by God] in faith,
Abraham was empowered (Greek is passive) by faith in God’s words. His faith derived from God. It was not Abraham’s faith per se that gave him strength; it was the God of his faith that gave him strength.
Faith can grow stronger. Faith is the proper answer to God’s promises.
giving glory to God,
Abraham acknowledged the manifestation of God’s capacity to do what He said He was going to do. He was fully confident in God’s power. Ultimately, we stand in a dependent relationship to God. Our faith glorifies God because it asserts His trustworthy attributes.
It is the God of our faith, not our faith per se, that gives us powerful faith.
We never glorify God apart from faith. Since God can never change His mind, we can count on Him. He does not promise one thing one day and the reverse of it the next day. Wistful hope is not faith. Truth faith is taking God at His word.
Faith is no vague notion. Genuine faith holds to God’s promises regardless of circumstance. It is God, not faith, that gives us strong faith. We have nothing except the commitments of God toward us. That is why we should develop faith as an ongoing attitude. This brings us into close fellowship with the Lord.
Many Christians vacillate between faith and doubt. When things go well, they trust God. When things go badly, they doubt Him. Everyone struggles with his or her faith, but a struggling faith is not the same as doubt. Weak faith capitulates to doubt. Testing will prove how strong our faith is.