Bible-believing Protestants use the Bible's own definition as to what must be believed unto righteousness. There is a foundation that has been laid, by God. No other foundation will support the 'building' that will remain into all eternity. As Paul stated, "For other foundation can no man lay than that is laid, which is Jesus Christ" (1 Corinthians 3:11). That is why the apostles preached Christ, and him crucified and risen. They taught that which is to be believed unto righteousness.
Therefore, their apostolic doctrine shows the foundation of God's righteousness to be that which the Book of the Revelation states - the New Jerusalem has a wall, "And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb (ch. 21 vs 14). This foundation is the doctrine of Christ, without which nobody can have either the Father, or the Son. But to abide in the doctrine of Christ is to have both the Father and the Son (2 John vs. 9).
"But God be thanked, that ye were the servants of sin, but ye have
obeyed from the heart that form of doctrine which was delivered you.
Being then made free from sin, ye became the servants of
righteousness" (Romans 6:17-18).
Nobody can believe unto righteousness without holding fast to the form of sound words, such as that which Timothy heard of the apostle Paul - 2 Timothy 1:13. Thus Paul curses all, angels or men, who pervert the doctrine of Christ (Galatians 1:8-12). The apostolic doctrine is called the evangel, or gospel: "I am not ashamed of the gospel of Christ: for it is the power of God unto salvation" (Romans 1:16). This is "the faith once delivered to the saints" - Jude vs. 3.
But once that gospel has been declared, hearers must exercise faith in it. Just hearing the biblical, apostolic doctrine of Christ is insufficient. Millions have heard it but have remained unmoved even though giving verbal assent to it. Millions more have heard it and responded by striving to earn, or merit, salvation, but they will never enter into God's "rest" because "the word preached did not profit them, not being mixed with faith in them that heard it. For we which have believed do enter into rest... they to whom it was first preached entered not in because of unbelief" (Hebrews 4:2-6).
Faith must be demonstrated - believing it so as to be transformed by grace, to show forth God's righteousness. And that is what Abraham did. The book of Hebrews has much to say about Abraham, and how it was his faith that pleased God:
"He that cometh to God must believe that he is, and that he is a
rewarder of them that diligently seek him... By faith Abraham, when he
was called to go out into a place which he should after receive for an
inheritance, obeyed; and he went out, not knowing whither he went. By
faith he sojourned in the land of promise, as in a strange county...
for he looked for a city which hath foundations, whose builder and
maker is God" (Hebrews 11:6-10).
That shows Abraham to have had faith in the glory of heaven, unseen; an even greater hope than that of the promised land on earth. This statement applies just as much to him as to others who became sons of God by faith - "For we through the Spirit wait for the hope of righteousness by faith. For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love" (Galatians 5:5-6). That is why Abraham pleased God long before he was commanded to be circumcised, he and his male household. He had faith in God right at the start of God calling him out of the land of his birth, to sojourn towards that promised land, and Abraham saw more than that which was only seen by his eyes. He had eyes of faith, for the Spirit of Christ lived in him thousands of years before Christ would be manifested on earth.
I have heard this taught in various Protestant sermons, and read it in many Protestant books, but a perverted gospel is creeping in, moreso these days than in decades past, for many teachers are being raised up, to tickle the ears of their hearers. Those teachers are wandering away from the apostles' doctrine, as they are from living by faith. The apostles warned that such a horrible state would obtain after their demise, false teachers with unrighteous ways, creeping in to lead the people astray with commands of men. But this answer gives something of the Bible's definition as to that which is to be believed unto righteousness.