Consider “Pascal’s Wager”: 100% certainty about God is not necessary in order to seek Him and to ask questions about religion. In seeking, we have little to lose, and eternity to gain. Simply getting someone simply to consider religious seeking is helpful. The evangelist could invite the atheist to say prayers, attend Mass, read the Bible or the Catechism, and to talk to Christians—all the while asking God for His grace to pave the way. When offering an argument, don’t set the bar too high; be modest in your claims and don’t overstate your case. Nudge people along gently and compassionately. One Good Reason is a helpful way to do this.
Also, don’t feel bad if some of these arguments are difficult for you to grasp. It takes work for anyone to understand them well, and some people will not be able to master them. However, not all the “arguments” are like this. The moral argument is easier than the contingency and teleological arguments, for instance. Most especially, though, we should remember that sharing a personal testimony and preaching the gospel are things that all of us can learn — and many times, can be the most fruitful of them all.