General revelation is truth about God revealed through general ways as opposed to special ways. To show his goodness, glory, and other perfections, God has generally revealed himself via the following routes: nature (Rom. 1:18-23; Psa. 19; Acts 14:15-17), conscience (Gen. 1:26-27; Rom. 2:14-16), humanity’s ontology (universal moral nature, inherent religious tendencies, and consciousness), and reason/logic itself. Can general revelation be the starting point for witnessing to the unbeliever? It depends on how much the unbeliever will grant. If the unbelieving individual grants the laws of logic, the analogical use of language, the basic reliability of sense perception and the law of causality, then of course discussions relating to general revelation can and should occur. Arguments built upon general revelation can be valuable tools in pre-evangelism. They remove intellectual difficulties people have with the faith.
Those that argue against apologetics built upon general revelation argue that it is arrogant or presumptuous to start at any other place other than the Scriptures because that denies the lordship of Christ over all things (e.g. John Frame). It sets up reason above the truth of God. I do not think this is the case for a couple of reasons. First, if it is sinful, we have cases where Paul is committing sin because he reasons apologetically without explicitly starting with the OT in Acts. He starts with creation and providence in Acts 17. Second, there is a difference between the order of being and the order of knowing that the presuppositionalist confuses. Of course, as Steve B. Cowan notes, the Bible is the Word of God (order of being) but such a belief does not help someone recognize the Bible is the word of God (order of knowing). Competing claims to inspiration among religions have to be decided on something other than their respective appeals to holy books. If an unbeliever will recognize and allow the aforementioned facts of existence, then apologetics built upon general revelation seem alright in their specific context.
If the unbeliever does not grant the assumptions of existence, then general revelation will not be very helpful. The cosmological argument will not be satisfying to someone who does not believe we can know causation actually occurs with any sort of probability. Design arguments will not convince someone who thinks the external world does not exist apart from their experience of it. After all, the design could be a facade? Arguments about the objectivity of morality will not work if someone is a moral relativist or nihilist. At that point, I think the presuppositionalist’s transcendental argument could be of immense value and worth. Unless God exists and this is his world, you cannot meaningfully know anything with certainty. You have no good reason to trust your reasoning faculties or any claims to truth whatsoever (including the ones your worldview makes). The unbeliever cannot consistently live in God’s world and deny his existence.
Can general revelation be the starting point? It depends on the level of rationality and unbelief of the individual. One thing is certain—general revelation does not save anyone. The problem of unbelief is not merely an intellectual barrier where all that is needed to cross is more facts. The will of an individual has also been damaged (Rom. 3:10-18). Responding to the Jewish leadership’s attack on his teaching (e.g. how Jesus knows what he knows), Jesus responds with, “If anyone’s will is to do God’s will, he will know whether the teaching is from God or whether I am speaking on my own authority. (John 7:17)” The problem in the text is not Jesus’ teaching; it is the hearers’ ability and desire to hear. Right knowledge rests on right willing. To know some things are true, we have to possess a desire to want to know them as such. This means that the truth [about Jesus, the object of faith] is self-authenticating—not with vicious circularity, as if it has no meshing-points with the external, examinable world (Does not Jesus himself invite us to believe on the evidence of the signs [John 10:38]), but in the sense that finite and fallen human beings cannot set themselves up on some sure ground outside the truth and this gain the vantage from which they may assess it. To borrow a phrase from Van Til, the spectacles of unbelief have to come off. The pure in heart see the purity of God. Those who see the purity of God are those to whom he gives a pure heart (Ezek. 36:26-27).