That's a very well put argument. So let's say you're right. What in the text indicates that this faith is aforeign faith as opposed to the inherent faith that you say everyone has? Where does Scripture speak of two faiths?
Romans 12:3ff deal with gifts so we then look at I Cor. 12 where Paul expounds on gifts too. To understand this (Romans 12) we look also at that (I Cor. 12).
Is R12 ligitimately related to ICor12? I believe they are.
Chapter 12
Diversity of Spiritual Gifts
1 Now concerning what comes from the Spirit: brothers, I do not want you to be unaware. 2 You know that when you were pagans, you used to be led off to the idols that could not speak. 3 Therefore I am informing you that no one speaking by the Spirit of God says, Jesus is cursed, and no one can say, Jesus is Lord, except by the Holy Spirit. 4 Now there are different gifts, but the same Spirit. 5 There are different ministries, but the same Lord. 6 And there are different activities, but the same God activates each gift in each person. 7 A demonstration of the Spirit is given to each person to produce what is beneficial:
8 to one is given a message of wisdom through the Spirit, to another, a message of knowledge by the same Spirit, 9 to another, faith by the same Spirit, to another, gifts of healing by the one Spirit, 10 to another, the performing of miracles, to another, prophecy, to another, distinguishing between spirits, to another, different kinds of languages, to another, interpretation of languages. 11 But one and the same Spirit is active in all these, distributing to each person as He wills.
1 Cor 12:1-11 (HCSB)
Chapter 12
A Living Sacrifice
1 Therefore, brothers, by the mercies of God, I urge you to present your bodies as a living sacrifice, holy and pleasing to God; this is your spiritual worship. 2 Do not be conformed to this age, but be transformed by the renewing of your mind, so that you may discern what is the good, pleasing, and perfect will of God.
Many Gifts but One Body
3 For by the grace given to me, I tell everyone among you not to think of himself more highly than he should think. Instead, think sensibly, as God has distributed a measure of faith to each one. 4 Now as we have many parts in one body, and all the parts do not have the same function, 5 in the same way we who are many are one body in Christ and individually members of one another. 6 According to the grace given to us, we have different gifts: If prophecy, use it according to the standard of ones faith; 7 if service, in service; if teaching, in teaching;8 if exhorting, in exhortation; giving, with generosity; leading, with diligence; showing mercy, with cheerfulness. Romans 12:1-8 (HCSB)
Next we have Paul touching on gifts in his letter to the Ephesians
Unity and Diversity in the Body of Christ
1 Therefore I, the prisoner for the Lord, urge you to walk worthy of the calling you have received, 2 with all humility and gentleness, with patience, accepting one another in love, 3 diligently keeping the unity of the Spirit with the peace that binds ⌊us⌋. 4 There is one body and one Spirit just as you were called to one hope at your calling 5. one Lord, one faith, one baptism, 6 one God and Father of all, who is above all and through all and in all. 7 Now grace was given to each one of us according to the measure of the Messiahs gift. 8 For it says:
When He ascended on high, He took prisoners into captivity; He gave gifts to people. 9 But what does He ascended mean except that He descended to the lower parts of the earth? 10 The One who descended is also the One who ascended far above all the heavens, that He might fill all things. 11 And He personally gave some to be apostles, some prophets, some evangelists, some pastors and teachers, 12 for the training of the saints in the work of ministry, to build up the body of Christ, 13 until we all reach unity in the faith (not as a verb but the body of truth of the reality of the body of Christ - my words) and in the knowledge of Gods Son, ⌊growing⌋ into a mature man with a stature measured by Christs fullness. 14 Then we will no longer be little children, tossed by the waves and blown around by every wind of teaching, by human cunning with cleverness in the techniques of deceit. 15 But speaking the truth in love, let us grow in every way into Him who is the head Christ. 16 From Him the whole body, fitted and knit together by every supporting ligament, promotes the growth of the body for building up itself in love by the proper working of each individual part.
Eph 4:1-16 (HCSB)
So what we intend to show here is solid proof that the applications of "faith" in these verses are post salvation and cannot be used to prove pre saving faith. ICor. 12 speaks of a "spiritual gift of faith" that some get and others do not and in R12 everyone receives a "measure of faith" that is related to humility in service of the out working of the gifts in the lives of the saved. There is a distinct difference and the gifts of Eph 4 emphasizing preaching, teaching, training etc. He specifically speaks to this more narrow set of gifts as he teaches Timothy where as in ICor 12 and R12 the emphasis is related to the total Body of Christ. It is where all saved members are gifted and addressed. Eph 4 is not the elimination of the others but the application of specific gifts in Pauls instruction to Timothy as a young pastor, more directly related to his calling.
Everything in these portions of Scripture is for the edification and training of the body of Christ post salvation.
How is it that a gift of faith to the one who has faith that was born out of the hearing of the gospel in which the person created in the image of God could exercise toward God's grace is now selected in limited numbers as members already in the body to receive a gift of faith that is distinctly not that which the believer already is has. This same application holds as well with R12. If we say that this specific gift faith and the allotment of measured faith are the same as believing faith we have a very big problem. At such a point if blended the evidence refutes.
So the question you ask while a probing one does not address the facts as we find them in Scripture. The question then, when we see the facts as we have given them, is to be framed differently. The question is then: Is there different kinds of faith? And the answer is yes because this is what the Bible indicates.