Thomas Boston in The Whole Works of Thomas Boston: Human Nature in Its Fourfold State and a View of the Covenant of Grace, ed. Samuel M‘Millan, vol. 8:
It is not only life that is promised, but life eternal, life for evermore, Psalm 133:3; which, from the moment it is given, shall never be extinguished, through the ages of time and eternity. In the style of the Scripture, eternal life is not restricted to the state of glory in heaven. But the life communicated to a sinner, in the first moment of his union with Christ, is eternal: it is the eternal life promised in the covenant, according to the Scripture, John 3:36, “He that believeth on the Son, hath everlasting life.” See chap. 5:24; 1 John 5:11, 12. Hence, from the promise of the covenant, “The just shall live by faith,” the apostle proves the perseverance of the saints, Heb. 10:38. A plain evidence, that perseverance in grace, in this our state of imperfection, is a part of the eternal life promised in the covenant, as well as heaven’s happiness. And thus the covenant-life extends to that which now is, and that which is to come, 1 Tim. 4:8.