Because
Romans 4:16
Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,
Romans 10:3
For they being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God.
Matthew 19:16-20
16
And, behold, one came and said unto him, Good Master, what good thing shall I do, that I may have eternal life?
17
And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.
18
He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness,
19
Honour thy father and thy mother: and, Thou shalt love thy neighbour as thyself.
20
The young man saith unto him, All these things have I kept from my youth up: what lack I yet?
John 6:27-29
27
Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed.
28
Then said they unto him, What shall we do, that we might work the works of God?
29
Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent.
Acts 16:30-34
30
And brought them out, and said, Sirs, what must I do to be saved?
31
And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
32
And they spake unto him the word of the Lord, and to all that were in his house.
33
And he took them the same hour of the night, and washed their stripes; and was baptized, he and all his, straightway.
34
And when he had brought them into his house, he set meat before them, and rejoiced, believing in God with all his house.
1 John 5:9-12
9
If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son.
10
He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son.
11
And this is the record, that God hath given to us eternal life, and this life is in his Son.
12
He that hath the Son hath life; and he that hath not the Son of God hath not life.
they stumbled
Romans 11:11
I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles, for to provoke them to jealousy.
Matthew 13:57
And they were offended in him. But Jesus said unto them, A prophet is not without honour, save in his own country, and in his own house.
Luke 2:34
And Simeon blessed them, and said unto Mary his mother, Behold, this child is set for the fall and rising again of many in Israel; and for a sign which shall be spoken against;
Luke 7:23
And blessed is he, whosoever shall not be offended in me.
1 Corinthians 1:23
But we preach Christ crucified, unto the Jews a stumblingblock, and unto the Greeks foolishness;