because.

Luke 3:8
Therefore produce fruit worthy of repentance. And do not begin to say to yourselves, 'We have Abraham as our father.' For I tell you that out of these stones God can raise up children for Abraham.

Luke 16:24, 25, 30
So he cried out, 'Father Abraham, have mercy on me and send Lazarus to dip the tip of his finger in water and cool my tongue. For I am in agony in this fire.' . . .

John 8:37-39
I know you are Abraham's descendants, but you are trying to kill Me because My word has no place within you. . . .

Philippians 3:3
For it is we who are the circumcision, we who worship by the Spirit of God, who glory in Christ Jesus, and who put no confidence in the flesh--

In Isaac.

Genesis 21:12
But God said to Abraham, "Do not be distressed about the boy and your maidservant. Listen to everything that Sarah tells you, for through Isaac your offspring will be reckoned.

Hebrews 11:18
even though God had said to him, "Through Isaac your offspring will be reckoned."