In case his explanation turns out to be useful (although it repeats much that has already been quoted in this thread by, e.g., Thomas Boston), Robert Shaw on the use of "condition" language for faith
http://www.reformed.org/documents/shaw/:
"In entering upon the exposition of this section, it is proper to remark, that, at the period when our Confession was framed, it was generally held by the most eminent divines, that there are two covenants connected with the salvation of men, which they called the covenant of redemption, and the covenant of grace; the former made with Christ from everlasting, the latter made with sinners in time; the righteousness of Christ being the condition of the former, and faith the condition of the latter covenant. This distinction, we conceive, has no foundation in the Sacred Scriptures, and it has long since been abandoned by all evangelical divines. The first Adam is said to have been a figure of Christ, who is called the second Adam. Now, there was not one covenant made with Adam, the condition of which he was to perform, and another made with his posterity, the condition of which they were to fulfil; but one covenant included both him and them. It was made with him as their representative, and with them as represented in and by him. In like manner, one covenant includes Christ and his spiritual seed. The Scriptures, accordingly, everywhere speak of it as one covenant, and the blood of Christ is repeatedly called "the blood of the covenant," not of the covenants, as we may presume it would have been called, if it had been the condition of a covenant of redemption and the foundation of a covenant of grace.—Heb. x. 29, xiii. 20. By the blood of the same covenant Christ made satisfaction, and we obtain deliverance.—Zech. ix. 11. We hold, therefore, that there is only one covenant for the salvation of fallen men, and that this covenant was made with Christ before the foundation of the world. The Scriptures, indeed, frequently speak of God making a covenant with believers, but this language admits of an easy explication, in consistency with the unity of the covenant. "The covenant of grace," says a judicious writer, "was made with Christ in a strict and proper sense, as he was the party-contractor in it, and undertook to fulfil the condition of it. It is made with believers in an improper sense, when they are taken into the bond of it, and come actually to enjoy the benefit of it. How it is made with them may be learned from the words of the apostle,—Acts xiii. 34: "I will give you the sure mercies of David,' which is a kind of paraphrase upon that passage,—Is. lv. 3: "I will make an everlasting covenant with you, even the sure mercies of David.' God makes the covenant with them, not by requiring anything of them in order to entitle them or lay a foundation for their claim to the blessings of it, but by making these over to them as a free gift, and putting them in possession of them, as far as their present state will admit, by a faith of his own operation."
The supposition of two covenants for the salvation of mankind sinners, is encumbered with various difficulties. One is obvious. In every proper covenant, there are two essential parts—a conditionary and a promissory. If, therefore, there be a covenant made with sinners, different from the covenant made with Christ, it must have a condition which they themselves must perform. But though our old divines called faith the condition of the covenant made with sinners, they did not assign any merit to faith, but simply precedence. "The truth is," as Dr Dick has remarked, "that what these divines call the covenant of grace, is merely the administration of what they call the covenant of redemption, for the purpose of communicating its blessings to those for whom they were intended; and cannot be properly considered as a covenant, because it is not suspended upon a proper condition." The Westminster Assembly, in this section, appear to describe what was then usually designated the covenant of grace, as distinguished from the covenant of redemption. But, though they viewed the covenant under a twofold consideration, as made with the Surety from everlasting, and as made with sinners in time, they certainly regarded it as one and the same covenant. "The covenant of grace," say they, "was made with Christ as the second Adam, and in him with all the elect as his seed.""
"That God "requires of sinners faith in Christ that they may be saved," admits of no dispute. The part assigned to faith, however, has been much controverted. Many excellent divines, in consequence of the distinction which they made between the covenant of redemption and the covenant of grace, were led to speak of faith as the condition of the latter covenant. But the term, as used by them, signifies not a meritorious or procuring cause, but simply something which goes before, and without which the other cannot be obtained. They consider faith merely as a condition of order or connection, as it has been styled, and as an instrument or means of obtaining an interest in the salvation offered in the gospel. This is very different from the meaning attached to the term by Arminians and Neonomians, who represent faith as a condition on the fulfilment of which the promise is suspended.. The Westminster Assembly elsewhere affirm, that God requires of sinners faith in Christ, "as the condition to interest them in him." But this is very different from affirming that faith is the condition of the covenant of grace. That faith is indispensably necessary as the instrument by which we are savingly interested in Christ, and personally instated in the covenant, is a most important truth, and this is all that is intended by the Westminster Divines. They seem to have used the term condition as synonymous with instrument; for, while in one place they speak of faith as the condition to interest sinners in the Mediator, in other places they affirm, that "faith is the alone instrument of justification," and teach, that "faith justifies a sinner in the sight of God, only as it is an instrument by which he receiveth and applieth Christ and his righteousness." As the word condition is ambiguous, apt to be misunderstood, and is frequently employed in an unsound and dangerous sense, it is now disused by evangelical divines."