I Samuel 12:22-24 (NASB)For the Lord will not abandon His people on account of His great name, because the Lord has been pleased to make you a people for Himself. 23 Moreover, as for me, far be it from me that I should sin against the Lord by ceasing to pray for you; but I will instruct you in the good and right way. 24 Only fear the Lord and serve Him in truth with all your heart; for consider what great things He has done for you.
The prophet Samuel, guiding the children of Israel, was asked by the nation to address God with their desire to have a physical king as did their neighbors. Samuel took this notion to God, who told the prophet to explain to them He was their king. No one could possibly want any better. Yes, they insisted.
Samuel addressed the crowd after a king was appointed to tell them they were appeased but not obedient to God. God was not happy. They were allowed a king but it was a rejection of God's desire for them and a lack of trust in Him. Samuel called to the Lord and thunder and rain came that day and destroyed the wheat harvest. The people saw their sin and cried out to the prophet to implore God for them. This is when Samuel made the statement cited.
This nation was called "the children of Abraham". Paul says Christians are the children of Abraham now in Christ. Galatians 3: 28-29 There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus. 29 And if you belong to Christ, then you are Abraham's descendants, heirs according to promise. Consider what great things He has done for you.