People will sometimes ask me how I know that God exists. When confronted with they query I could dive into the cosmological, teleological, or moral arguments, but while those are all solid arguments for the existence of a deity, none of them fully encompass who God really is, and none of them can really prove anything. Logic will only get you so far, after all as C.S. Lewis pointed out: "You cannot prove rational intuition by argument because argument depends on rational intuition." So therefore how can you know for sure that God exists?
To answer this question let us think back to before you first were. While you were still in your mother's womb you could not prove that there was such a thing as "mom", nor could you prove that there was life after birth. All that you knew was darkness. When the time came for you to be born you were suddenly and gloriously transformed from the world of "darkness" (literally) to the world of light. Still you were not fully aware that your mom existed because you were too undeveloped to be capable of such high thought, but you knew that there was more. After just a few years of development had anyone asked you if your mom existed you would immediately say (were you capable of communication) "yes of course!" and how would you know? You experienced her on a daily basis. From there as you got more and more developed physically and mentally* you would become more and more sure that your mother exists because you know her. Not do you just see her, feel her, or communicate with her, you actually know her on a intimate level. You know what she wants, how she wants it, and why she wants it that way. You know her will.
Now the cool part of this is relating this to the Christian life. When we were still yet unsaved we could not prove anything beyond the physical. We floated in a world of darkness and despair. How could we know if there was such a thing as "God" and how could we know if there is life after birth? When we were born into saving faith (by the grace of God, not by work of our own, just like physical birth) we suddenly were moved from the kingdom of darkness to the kingdom of light. No longer were we blinded, but we also had no means of comprehending what we had just experienced. We could not even fully prove that God is real, we just knew there was something more. After we started to develop in the faith we could slowly begin to have full confidence that God exists because we experience him on a daily basis, were anyone to ask us if he exists we could not necessarily prove it, but we could know for sure that he did. From there as we became more and more developed in the knowledge of God and maturity of faith** we began to become more and more sure that God was real because we had begun a relationship with him. From that point until death takes away all barriers we will be in a process of getting to know God more and more deeply, and although we will always be progressing in this knowledge we will never reach the bottom of the well, there will be no end to what we can know about God. We will know what he wants, how he wants it, and why he wants it that way. We will know His will.
There is a reason that Jesus used birth as an analogy for salvation. What then is the point of all of this? We do not have to be afraid that someone will come along and prove to us that God does not exist because just as someone can't make me disbelieve that my mother exists, there is no way to disprove that God exists in my mind because I have relationship with him. If you get to know God through talking and listening to him in prayer and reading the 66 book love letter he wrote you you will have confidence. Know God.
*This is not a choice that we can make (we will grow up), we just grow despite what we would desire, unless we intentionally destroy our body through deprivation of food. The extent to which we grow and the health of our growth is greatly affected by how we treat our body and what food and exercise we give it.
**Again this is not a choice that we can make (we will be sanctified), we grow despite what we would desire, unless we intentionally destroy our soul though deprivation of spiritual food (like the Word, and fellowship). The extent to which we grow and the heath of our growth is greatly affected by how we treat our soul and what food and exercise we give it.
Recommended reading on the topic:
Also though I haven't personally read it yet John Owen's Communion with God is supposed to be good.