The following is provided so that you can confidently and effectively speak to the world using the world's logic.
If God's only limitation is being invited, then we should be trying to get Him invited. Plant the seed (1 Corinthians 3:5-8). And we cannot judge what soil people are (Matthew 13:1-9).
Speaking to non-believers is an art. Luckily many of us were them not long ago. These arguments were gifts along our walk and have proven effective with most anyone open to discussion. Immediate agreement is unlikely. The goal is simply planting a seed or idea that is unlike anything they've heard.
People think humans recently advanced enough to understand God is just a construct. Yet for as long as God has been in human history, men have sought excuses to deny Him. This is not unique, just prideful hubris. Has society's systemic retreat from God left the world a better place?
Arguably it looks foolish if you DON'T believe in God, here are some simple reasons why.
(Establish common ground, with a strange solution to a common problem that has left both science and biblical scholars unsatisfied.)
Young vs. Old Earth
Is Earth 4.543 billion years old or 6000?
When Adam and Eve were made in a few days, how old were they? Let’s say 20. So they are both 20 years old and a few days old. Two planes of time, two answers, both correct. God can create things imbued with an apparent age, He is infinite.
(Try to be as simple as possible. Occam’s razor)
If you hear hoofbeats, do you think unicorn, zebra, or horse? Horse is the simplest answer. Life just makes more sense with God in it.
There is a logical principle known as Occam's razor, where, all things being equal, the correct answer to a question is the simplest one. The simplest answer is the answer that involves the fewest assumptions.
If you hear hoofbeats...
God not existing invokes many assumptions: the world and universe inexplicably came into existence, the Earth was capable of sustaining life and continues to just by chance, life arose on this planet on its own somehow, simple life evolved into millions of complex forms of life including you, humans became self-aware and unlike anything else, we imagined God and never grew out of it (all the history). You are here simply because you exist, and are but a speck of cosmic dust in an impossible-to-comprehend universe, you are merely a random fluke in a random universe, and life holds no ultimate meaning.
The existence of God invokes fewer assumptions: God made everything and is beyond our comprehension. God sent Jesus as a way for us to comprehend.
God existing IS the simplest answer.
(Ask if they pray, because it is stupid not to. If they do, you can ask if they pray to Jesus (Christian or spiritual). If they don't pray, challenge them with gambling a thought.)
Kindly and calmly ask if they pray.
Us: Do you pray?
Us: Why not?
Us: What is the cost of a prayer?
Them: Nothing or Time just not "thought"
Us: No, everything costs something and time will not change or be sacrificed, you can use your hands while you Pray. The cost is a thought, your most abundant resource in this life. Now what is the potential reward of a prayer?
Us: The answer is infinite, because the possible reward is that God will answer. A gamble we gladly take.
Sometimes it takes a lot of prayer to show God we seriously want something, so have some patience.
If they do not care enough about themselves to potentially waste a thought, then maybe their love for their family or friends is enough to pray for them. If they refuse to even do that, you could call them selfish, but that conclusion is implicit in the question, and directly calling them out would not be acting with love.
We enjoy explaining how God is asking so little of us while we give so much of ourselves to everything else. Bizarre contradiction.
P.S. – If anyone really questions God’s existence, look at all of Montana!
Humility is a must
Every Christian has faced moments where their faith, Bible, or Savior is attacked. Anti-Christian attitudes may originate from Christians forgetting what Jesus Christ taught, or failing to act in a Christlike manner. Attacks are simply how some people ask questions.
We are all born wondering who we are, why we are alive and aware, and what happens when we die. Only God can answer these questions. If we claim to know Jesus, then at the very least we should love (greatest commandment) and be self-sacrificing (illustrated by crucifixion). In a very simple sense, love is positive, so we should act positive around non-Christians. Any negativity will be seen as being unhappy with this God we claim to know, or being unhappy with ourselves, casting doubt on God's very existence.
Negative: Eliminate (insert sin of choice) from your life to get closer to the Almighty. Repent!
Positive: Jesus loves you and wants the best life for you. Being closer to Him will help to actualize a better life.
People say a lot of nasty things about Christians.
Christianity is so large (over 2 billion people) and ancient that it is almost as representative of human nature itself as it is of the tenets of Christian faith. The law of large numbers says that the larger a sample gets, the more representative it is of the true population. If you get a big enough sample, it will represent the population (good and bad). Anti-Christians continually cherry-pick and highlight certain 'bad examples' to tarnish Christianity as a whole.
The current situation could be considered analogous to demonstrations and rioting against police brutality. The majority of police officers are genuinely good people, many of whom are Christians, but from personal experience and anecdotes, it's clear some police are just wicked-hearted weaklings. There are brave police officers that are not scared of getting hurt in order to avoid taking a life. There are other cops who look at suspects or criminals as being less than human, and see their lives as expendable. The former sees the latter, but does nothing; widespread anger is the result.
The phenomenon of police brutality is not really a police problem so much as it is a societal problem, where some people take an opportunity to hurt others in order to feel better about themselves. A customer might blow up at a customer service representative because they have 'the right to be angry,' a diner might make a waiter feel awful for forgetting something or being absent too long, and in general, some people relish hitting anyone dramatically smaller than them. The victims in these examples cannot stand up for themselves, but we can when we see it. I think Jesus would, and he would do so calmly and respectfully.
If these wicked people exist among the human population then you will see them emerge within any group that becomes large enough. This is the law of large numbers. Many people are not seeing the true nature of the problem, since they are too focused on the actions of a few that they extrapolate to represent the whole population. That's why some may perceive Christians as being close-minded, hypocritical, or what-have-you, or police as being trigger-happy. The fight should be against Evil and evil people. There are a lot of good cops who have no tolerance for the wicked ones. Instead of demonizing all police and demanding the entire police force be dismantled, good cops should be empowered to keep things in check. Similarly, using a few cherry-picked examples to demonize Christianity as a whole is a dishonest tactic that is not used in service of positive change, but rather, as a means to delegitimize Christianity in the public eye.