Just like Abram, we are justified (declared righteous) by God because we have believed Him in the promise of the Son. We believe because we have been effectually called with a holy call according to the purposes of God, not the purposes of any man.
After these things the word of the LORD came to Abram in a vision, saying,
“Do not fear, Abram,
I am a shield to you;
Your reward shall be very great.”
2 Abram said, “O Lord GOD, what will You give me, since I am childless, and the heir of my house is Eliezer of Damascus?” 3 And Abram said, “Since You have given no offspring to me, one born in my house is my heir.” 4 Then behold, the word of the LORD came to him, saying, “This man will not be your heir; but one who will come forth from your own body, he shall be your heir.” 5 And He took him outside and said, “Now look toward the heavens, and count the stars, if you are able to count them.” And He said to him, “So shall your descendants be.” 6 Then he believed in the LORD; and He reckoned it to him as righteousness. Genesis 15:1-6 (NASB)
In Genesis 15:1-6 (above), God gives us through His prophet Moses probably one of the most important truths found in His Word. This truth is all wrapped up in God’s Sovereignty. In v1 we read that through a vision given to him by God, Abram learns that God is his shield and that he will receive a great reward. In vv2-3 we witness Abram’s fear and his plea for an heir from his own loins. His focus is temporal, but he recognizes that unless God ordains it, there will be no child of his own. Here we see that Abram is no different than us. When he is in control then he is dominated by fear and unbelief. However, in vv4-5 God gently deals with Abram’s unbelief and fear by assuring him that he would indeed have a son and that his descendants would be innumerable. From this we learn that God’s promises are eternally focused even if they have temporal elements. We are called to believe based on His character not our circumstances. In these two verses, we have God’s effectual call of Abram. In v6 we read that Abram believed the Lord, and God counted it to him as righteousness. As a result of every effectual call by God, the one called by God believes Him and as a result He justifies them. They are saved by grace through the saving faith that accompanies every effectual call.
However, Christians are like Abraham in another way. We are living in between the promise and the fulfillment just as He was. Therefore, we live in an environment that is unredeemed and hostile to God’s truth and especially the genuine gospel. We struggle with becoming too focused on the temporal and this results in us becoming all bound up in fear and unbelief. When we attempt to draw closer to God and become the Christians we are called to be, we are attacked and accused by the enemy who attempts to make us believe that we are not saved by grace through faith like Abram, but by our faithfulness instead. If we listen to this then we can become quite discouraged because we are far from perfect. However, God is good to those whom He has called.