“Three times in the New Testament (2 Cor. 1:22; 5;5; and Eph. 1:14), it is stated that the experience of the Holy Spirit coming upon us is the earnest of our inheritance. This is a reference to the Old Testament practice of receiving a very small portion of that which has been purchased or promised as a token and pledge that the full purchase will be delivered in due time. By this means it is indicated to us that this glorious experience, which we usually call the Baptism in the Holy Spirit, is but a sample and a foretaste of that effulgence of glory which will be ours at the coming of our Lord.”
Source: Ralph M. Riggs; The Spirit Himself (1949); Chapter 9: Baptism in the Holy Spirit, Symbols Describing It; An Earnest; p. 74-75.