What is "Necessary Inference?"
"Necessary inference" can be defined as "a conclusion militated by reason and logic applied to known facts." It is simply a truth that is not directly or expressly stated, but which must necessarily follow as a logical conclusion from what is stated. The conclusion drawn from the facts is irresistible.If (for example) there is snow covering the ground in the morning, one may infer that the temperature was below 32 degrees during the night. This would be a necessary inference. We use such reasoning in everyday life so often that we usually don't even realize we are doing it. If I tell you my age, that is a direct statement. But if tell you the date of my birth, you may reason to the same conclusion. This is all that is meant by "necessary inference": a conclusion that is not directly stated but necessarily follows from other known facts. Whether a conclusion is stated directly or indirectly, either way it is just as true.
Now, it is evident that Scripture sanctions the use of reasoning in our coming to believe what is true. We read that Paul "reasoned from the scriptures" to prove that Christ must die and rise again, and that Jesus is the Christ (the "scriptures" from which Paul reasoned were the Old Testament - cf. Acts 28:23). But what Old Testament passage directly or explicitly states that Christ must rise from the dead (without reasoning to conclusions)? What passage directly or explicitly stated that Jesus of Nazareth would be Christ? Old Testament prophecy definitely shows that Jesus is the Christ, but this requires taking passages and reasoning to the necessary conclusion that Jesus would rise from the dead and is the Christ. Note that this was the method Paul "customarily" used to persuade people (Acts 17:2).
Objection 2: "Universal Salvation Wouldn't Be Fair"
For either the Lord does not care for all men (and this is the case either because he is unable- which is not to be thought, for it would be a proof of weakness - or because he is unwilling, which is not the attribute of a good being - and he who for our sakes assumed flesh capable of suffering, is far from being luxuriously indolent) or he does care for all, which is befitting for him who has become Lord of all. For he is Savior; not of some, and of others not. But in proportion to the adaptation possessed by each, he has dispensed his beneficence both to Greeks and Barbarians, even to those of them that were predestinated, and in due time called, the faithful and elect...And it cannot be said that it is from ignorance that the Lord is not willing to save humanity, because he knows not how each one is to be cared for. For ignorance applies not to the God who, before the foundation of the world, was the counselor of the Father...Nor does he ever abandon care for men, by being drawn aside from pleasure, who, having assumed flesh, which by nature is susceptible of suffering, trained it to the condition of impassibility.
And how is he Savior and Lord, if not the Savior and Lord of all? But he is the Savior of those who have believed, because of their wishing to know; and the Lord of those who have not believed, till, being enabled to confess him, they obtain the peculiar and appropriate boon which comes by him.
The Stromata, Book VII, Chapter II.—The Son the Ruler and Savior of All