40+% of Jews world-wide are agnostic or atheist, and this is especially true among Russian emigres. Here in Philadelphia with a Russian-speaking population of about 70,000, at least 70% of Russian speakers are Jews. Years ago Communist Russia taught something called "Scientific Atheism" based on the presumption that atheism was the foundation for empirical science and that religious faith was simply superstition. If you wanted to get anywhere in Russian academia or be "upwardly mobile" in the Party or the Institute or the factories, you kept your religion to yourself. Many of these US emigres are still atheist/agnostic because they've never had a dialog with a Christian who knew what he was talkiing about. But even among Americans in general, agnosticism and atheism is widespread.
There are MANY fine books written on how to share your faith with "A&A" people. But in case you don't have time for a book, here are some basic rebuttals condensed into a few talking points.
1) Reasoning with Agnostics: If they object to taking a blind "leap of faith" (as they think you've done!), then tell them they've already taken that leap by concluding that no conclusion can be reached about God. The point you should make with them? Their view itself is a religious faith commitment.
2) They assume agnosticism is open minded. Challenge that assumption on the basis of Romans 1:18-23. Ask them: "Wouldn't you be more inclined to be biased against faith in an all-powerful Judge of deeds and thoughts who brings mankind to a Day of Judgment? Ask: "How can you be sure that you wouldn't be prejudiced (and not open-minded) against such a conclusion, and therefore avoid giving it due consideration?"
3) Show them John 3:19 and following verses. Even if they discount your use of the Bible, certain verses contain "cut-to-the-heart" timeless truths that even the un-churched can see. For example: "Light is come into the world, but men loved darkness rather than light, because their deeds were evil."
4) Ask them if they are open-minded or narrow-minded. Agnostics/Atheists believe they've arrived at their views because they're open-minded. They aren't, but what you're doing here is getting a commitment from them to keep an open mind. Ask: "Are you willing to change your mind if I could show you that belief in the biblical God is not only possible logically, but is in fact essential for a civilized society? (In other words, "Are you willing to change your mind if it can be shown that atheism/agnosticism doesn't make logical sense?)
5) Apart from there being a God (i.e., an ultimate Law-Giver), there is no basis for any appeal to morality, truth or ethics. Why? Because if ours is a world and universe of random-chance (as "A&A" people believe) then how can we appeal to a transcendent, permanent moral order, or use terms like "justice vs. injustice" or "evil vs. good", etc.? For example ... why is what ISIS doing to it's victims called "evil". Why isn't it simply called "painful" ?
6) Listen for their use of words like "good/evil", "right/wrong" in their argument, then challenge them on that by asking: "How can you appeal to transcendent, ultimate moral law without presupposing an ultimate, transcendent Law-giver? If there isn't one, then there's only three reasons for morality: a) tradition b) survival of the fittest c) personal preference. (Give examples of where each of these leads.)
7) What if they say to you: "The existence of evil upon the innocent proves there is no God!"? Answer this way: "If you don't presuppose God to begin with, then stop using the word "evil", because you have no grounds for appealing to a ultimate moral order in your agnostic/atheistic universe of random-chance."
8) So then, if there is a God, what sort of God would permit evil upon the innocent? (Only three choices:) a) He's righteous but not all-powerful to stop it; b) He's all-powerful to stop it, but not righteous enough to do so; c) He's both, but you're not able to understand how he can be both.
9) Ask this: "Jesus was an innocent man, subjected to unjust evil. What explanation did He give? Would you like to see from Scripture?"