The indirect witness is an inference from the discerned presence of the fruit of the Spirit, love, joy, peace, etc., and it follows the direct witness in the order of time, "because," says Wesley, "in the nature of the thing, the testimony must precede the fruit which springs from it." The voice of the Spirit within the believer is to all who know God the most real of all realities. It is sometimes called a seal which secures, authenticates and appropriates.
The Holy Spirit is God's seal. "Ye were sealed with (not by) that Holy Spirit of promise" (Eph. i. 13). Another metaphorical designation of the witness of the Spirit is "the earnest of the Spirit." The earnest is derived from mercantile usage traceable through the Romans and Greeks to the Phoenicians, the founders of commerce. It assures the fulfillment of a promise as a part of the purchase money paid in advance to bind the bargain, or as an installment of a servant's wages paid at the time of hiring, obliging the servant to render the service and the master to pay the rest of the wages after the work has been done. It places both parties in a position to enforce the contract. The buyer, if he does not take the goods, forfeits the money advanced, and the servant who fails to render the service must refund the earnest which he has taken. The master who repents of his bargain must lose the wage advanced, and the merchant who withholds the goods because the market price has risen, or for any other reason, must repay the money advanced. The phrase "earnest of the Spirit" occurs only twice in the New Testament. Grammatically "the Spirit" is in apposition with "earnest," meaning that the pledge consists in the Holy Spirit bestowed upon the believer and dwelling in his heart. "And gave us the earnest of the Spirit in our hearts" (II Cor. 1. 22). There is no hint here of the time when the full wage will be paid, although in Eph. i. 14 it is "until the redemption of the purchased possession." Those who take a narrow view of present Christian privilege and put the fruition of the promises after death interpret the earnest only of the fulness of joy in heaven. But I believe that it is a pledge and a foretaste not only of heaven hereafter, but of a present heaven attainable by faith – even the fulness of the Holy Spirit.
— The Gospel of the Comforter Chapter 16.