When God makes a promise, best believe it is going to come to pass and be fulfilled. When God says something is going to happen, rest assured that it will take place. When God says He's going to do something, though it may take time, be confident that it will get done. Since God is eternally almighty, He is able to anything and everything He wants. Since God is faithful, He will execute His Word to 100% fulfillment in His perfect timing. It doesn't matter what the promise is and what the variables are - God will make it come to pass. It doesn't matter what the opinions of people are and the present circumstances are completely irrelevant. God will always do as He says!
In Genesis 21:1-7 the Bible describes an instance where God fulfilled one of the most important promises in scripture. In Genesis chapter 12 God promised Abraham that he would be a great nation. In order to be a great nation, Abraham needed to have children - many children. The scriptures stated that Abraham was childless at 75 years old in Genesis chapter 12. The Bible then shows that Abraham and Sarah got antsy and impatient with God. They figured since it had been 10 years that God had maybe forgot about His promise, or had rescinded on His promise because of sin, or maybe even that the circumstances were too difficult for God. Therefore, the Bible explains that Abraham and Sarah sought to take matters into their own hands instead of trusting in God's Word. Abraham had a child with Hagar who was named Ishmael, and life became much more complicated.
The Bible explains that 14 years after that, God called out to Abraham in order to call him to repentance for his unbelief. God restated His promise to make Abraham a great nation. God assured Abraham that he would have a son with his wife, Sarah, and that son would be named Isaac. Later in scripture the Bible explains that Jesus visited Abraham to reassure him again that he would have a son named Isaac in a year's time. Though 24 years had passed since God originally made the promise, God was emphatically assuring Abraham that the circumstances of age would not be able to restrict His ability to fulfill His promise.
By the time the Bible progresses to Genesis 21:1-8, 25 years had elapsed since God first made His promise to make Abraham a great nation. Nevertheless, Genesis 21:1-8 explains that Sarah indeed gave birth to a son named Isaac. Though Sarah was 90 years old, she was able to give birth and nurse a child - just as God had promised. When the child was born, Jesus went back to visit Abraham and Sarah - just as God had promised. Though Abraham was 100 years old, and as the Book of Hebrews puts it, "As good as dead," Abraham had a son - just as God had promised. Abraham and Sarah named their son Isaac - just as God had promised.
The Bible is clear to show that God will get done the things that He says He will get done. Though His timing might not meet the expectations or ideas of the people, God's Word will be fulfilled. It is true that God waited 25 years to do something that He could have done 25 years earlier. However, the scriptures reveal that God was trying to prove a point. God wanted to prove that circumstances can't affect His ability to do as He promises. God wanted to prove that the fulfillment of His promises is not dependent on the abilities of humans since Abraham and Sarah had a child WELL past the age of childbearing. God wanted to prove His glory, His power, His faithfulness, and His grace.
The Bible describes that Abraham and Sarah were in awe over what the Lord had done. They were excited to give their child the name "Isaac," which means "Laughter," because the circumstances of their life had made them laugh. It was almost too hard to believe. Yet the truth of God's ability and faithfulness is tangible. The fact that Abraham and Sarah had a child to raise was proof of God's faithfulness. Just the same, the Jewish people today - the descendants of Isaac - are proof of God's faithfulness. There have been billions of people who have come from the loins of Abraham and Isaac, just as God had promised. One can look at the nation of Israel today as proof of God's faithfulness. Therefore, since God proved Himself faithful to His promises then, He is faithful to His promises just the same today because He does not change! Praise the Lord!