We cannot believe that such could happen

God often stretches us just a little farther than what we think is possible. That is what he did with Abraham, or as his name was at first, Abram. First God appeared to Abram (the name means exalted father), and made great promises to him.

“1 … “Get out of your country, and from your relatives, and from your father’s house, to the land that I will show you. 2 I will make of you a great nation. I will bless you, and make your name great. You will be a blessing.””
Gen 12:1-2

So Abram left at age 75, and Sara his wife was 65, to obtain the promise that he would become “a great nation.” It was actually kind of late for him to have children when he was called, but Abram believed God and it was counted to him as righteousness (Gen 15:6, Rom 4:22). Later the promised was expanded to include a certain land, but … still no children were coming to Abram. Abram was even promised that his descendants would be a countless as the stars of heaven in Genesis 15, but … still no children were coming to Abram.

Then Sarai thought of a way to maybe “help” God.

Perhaps Sarai could have a child through her personal slave Hagar, and perhaps that was what God intended. So they tried that and Abram did finally have a child by Hagar at age of about 85. It was a son, Ishmael. So finally Abram did have a child.

Finally God appeared to Abram again at age 99 in Genesis 17. God told him,

“… “I am God Almighty. Walk before me, and be blameless. 2 I will make my covenant between me and you, and will multiply you exceedingly.””
Gen 17:1

Then God went on to change his name to Abraham (which means father of a multitude) and said, “… for the father of a multitude of nations have I made you.” Gen 17:5.

Then came one of the real tests of Abraham’s faith.

At age 99 God told Abraham that He would make Abraham’s promised offspring to come from Sarai. Abraham was astonished and he pleaded, Can I really have a child at age 100, with Sarah being 90 years old? O that Ishmael might live before You, Gen 17:17. But God said, No, the promises will come through a child from Sarah, Gen 17:19. And Abraham believed it!

There were more tests to come, but Abraham did not waver in faith, and in the end had a child at age 100 by Sarah, and did receive the promises.

Similarly God tests our faith in what He says.

Will there really be one evil man who will be worshipped by the entire world (2Thessalonians 2, and Revelation 13)? Why that is unbelievable! And that man will die and be raised from the dead, Rev 13:3? Why that is unbelievable! And this man will overcome the church world-wide, Rev 13:7? Why, that is unbelievable? So we reject what we have in fact, truly seen in Scripture.

The issue is not believing God.

Scriptures are from the World English Bible (WEB), a copyright free revision
of the original ASV American Standard Version 1901