Milton is explicit in the Christian Doctrine: "God foreknew those who would believe, that is, he decreed or approved that they alone would be those for whom in Christ he should have regard—all, certainly, if they believed " (the new Oxford edition, 8.1:87; my emphasis).  The point is clear in the Latin ("omnes utique si credidissent").  The passage is in the Yale edition at 6.181-82 and in the Modern Library edition—slipping in a plug here—on pp. 1164-65).  Milton emphasizes the universality of offered grace a few pages later: "[I]f God rejects no one except the disobedient and the unbeliever, surely he imparts grace—if not equal, yet sufficient—to all, by which they may be able to arrive at recognition of the truth and salvation.... The reason, therefore, why God does not deem all worthy of equal grace is his own supreme will; the reason, however, for his deeming all worthy of sufficient grace is his justice" (my emphasis again, Oxford 8.1:101-03; in Yale, 6:192-93; in MLM 1168-69).