The following guidance comes from the Apostle Paul in Romans 9
1. TRUSTWORTHINESS OF SCRIPTURE
When skeptics question the trustworthiness of Scripture (has it come to pass?, Can God's word be trusted?, etc.), remember that often there are deeper issues surrounding this question when raised by skeptics.
First, communicate how you feel concerning the condition of the lost. Whereas Christians are often accused of indifference toward the condition of unbelievers, this is not reflective of the heart of Christ. Note how Jesus mourned over Jerusalem, how Moses asked that his name be blotted out in Exodus 32, and how Paul in Romans 9 was filled with great sorrow and unceasing anguish in his heart over the condition of his lost relatives and countrymen(Romans 9). Communicating one's response and feelings will not only set forth the truth, but also eliminate unnessary distractions which may stand in the way of others hearing and accepting the gospel.
Next, deal with the questions where others question either the trustworthiness of Scripture or Jesus as the Messiah. Typically, one can show the problem is not with God's faithfulness or the trustworthiness of His Word, but unbelief on the part of skeptics and their failure to either interpret Scripture correctly or receive it.
Finally, point out that the skeptic's own unbelief results in their failure to receive the inheritance offered in the promise.
2. JUSTICE OF GOD
When skeptics decry God as unjust because of his election to salvation of some but not others, point out that "mercy" is the basis of God's salvation. The nature of mercy is not conducive to issues of "fairness" or "justice", but operates differently.;
3. PERSONAL RESPONSIBILITY
When skeptics in response to God's purposes in election ask "Why then does God blame us?", responsd by pointing out 1) To denounce God as unjust is not only to try to shift blame to God but to usurp created order and make God accountable to man; 2) The Freedom and Authority of God to do with creatures what he will; 3) The fact that God treats creatures only as their sins deserve (or better!); Even when God judicially hardens the hearts of individuals, he does not violate the freedom or will of the individual, but only provides greater opportunity for the nature of their heart to be more openly revealed.
But note, never end the discussion at this point, After dealing directly with the faulty error of skeptics, turn the discussion again toward God's sovereign grace toward those who are objects of his mercy.
CONCLUDING GUIDANCE: Persuade others with the gospel. "What then shall we say?" v. 30 “What then shall we say?”
If God’s Word is INFALLIBLE (trustworthy & will surely come to pass)
If God’s Ways are characterized by FREEDOM (he is not bound to respond any certain way, but has chosen to grant salv. to those who will believe on his name)
If God’s Wrath & Power & Being is Formidable (He is Beyond Accountability to Us, but has made the riches of his glory known to the objects of his mercy)
WHAT THEN SHALL WE SAY? HOW SHALL WE RESPOND?
At this point, as it has been written, while some will stumble and fall, the one who trusts in Christ will never be put to shame.