However, between Romans 5 and the verses from Ephesians 2:8-9 posted by mommd, the principle is there.
IF it is not by faith alone, but by faith plus works, you have to ask the following questions:
What works do you have to do?
How many do you have to do?
Salvation now becomes incumbent upon your actions and not the one time sacrifice of Christ on the cross.
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However, if you believe in the Trinity you will not find that in Scripture either, yet I don't know of a Christian who denies the Trinity.
The principle lesson here is something doesn't have to be explicitly said in Scritpure; yet from Scripture the principle can be learned.
What works do you have to do?
Both Jesus and St. Paul are abundantly clear on the things we must do and must avoid to inherit the kingdom of God. St. Paul even provides lists of them, most prominently in Galatians.
How many do you have to do?
God doesn't give us numbers or amounts, so we don't have to worry about it. It is not our concern.
Salvation now becomes incumbent upon your actions and not the one time sacrifice of Christ on the cross.
That is a false dichotomy. It is not either or, it is both and, as Scripture abundantly makes clear.
You mean like this?
But he who made you without your consent does not justify you without your consent. He made you without your knowledge, but He does not justify you without you willing it. -- Augustine of Hippo, Sermon 169