All true Christians are believers, but not all believers are Christians.
That is in the Bible. Abraham wasn’t a Christian. Christianity belongs to the time following the ministry of the Lord Jesus, the descent of the Holy Spirit, and the inauguration of the church. That’s when the disciples became Christians. You don’t read the word “Christian” in the Old Testament, nor is a Christian described. What you have is godly or ungodly Israelites; those who believed God and those who didn’t; the wicked and the righteous in Israel — and of course some Gentiles saved as well.
The position from which a godly Jewish believer would look at things and the stance of an equally godly Christian looking at things are quite different.