The condition of the covenant of grace, so far as adults are concerned, is faith in Christ. That is, in order to partake of the benefits of this covenant we must receive the Lord Jesus Christ as the Son of God in whom and for whose sake its blessings are vouchsafed to the children of men. Until we thus believe we are aliens and strangers from the covenant of promise, without God and without Christ. We must acquiesce in this covenant, renouncing all other methods of salvation, and consenting to be saved on the terms which it proposes, before we are made partakers of its benefits. The word “condition,” however, is used in two senses. Sometimes it means the meritorious consideration on the ground of which certain benefits are bestowed. In this sense perfect obedience was the condition of the covenant originally made with Adam. Had he retained his integrity he would have merited the promised blessing. For to him that worketh the reward is not of grace but of debt. In the same sense the work of Christ is the condition of the covenant of redemption. It was the meritorious ground, laying a foundation in justice for the fulfillment of the promises made to Him by the Father. But in other cases, by condition we merely mean a sine qua non. A blessing may be promised on condition that it is asked for; or that there is a willingness to receive it. There is no merit in the asking or in the willingness, which is the ground of the gift. It remains a gratuitous favour; but it is, nevertheless, suspended upon the act of asking. It is in this last sense only that faith is the condition of the covenant of grace. There is no merit in believing. It is only the act of receiving a proffered favour. In either case the necessity is equally absolute. Without the work of Christ there would be no salvation; and without faith there is no salvation. He that believeth on the Son hath everlasting life. He that believeth not, shall not see life, but the wrath of God abideth on him.
—Charles Hodge, Systematic Theology (Oak Harbor, WA: Logos Research Systems, Inc., 1997), 2.364–65.