Herman Witsius, The economy of the covenants, volume 1, pp. 367-368:
XII. Besides, when God proposes the form of the covenant of grace, his words, to this purpose, are mere promises, as we have lately seen from Jer. xxxi. and xxxii. Our divines therefore, who, in consequence of the quirks of the Socinians and Remonstrants, have learned to speak with the greatest caution, justly maintain that the gospel, strictly taken, consists of pure promise of grace and glory.
XIII. And indeed, if we are to take the promises of the covenant of grace altogether without exception, we could not, so much as in thought, devise any thing in us, as the condition of these promises. For whatever can be conceived as a condition, is all included in the universality of the promises. Should God only promise eternal life, there might be some pretence for saying, that repentance, faith, and the like, were the conditions of this covenant. But seeing God does, in the same breath, as it were ratify both the beginning, progress, uninterrupted continuance, and, in a word, the consummation of the new life; nothing remains in this universality of the promises, which can be looked upon as a condition of the whole covenant.