Christian faith does not oppose reason, but embraces it: faith is in the far side of reason. Below you will find two simple logical demonstrations of the existence of God.
The argument from Absolute Truth
We start with the following definitions.
Definition 1: an absolute truth is any truth that transcends space and time; that is, it existed before the beginning of time and space and will exist after time and space have ceased to exist.
Definition 2: God is an immaterial reality who holds in Himself the cause of His own existence and, among other attributes, He is intelligent.
Now, we will claim that the absolute truth does not exist and perform a reductio ad absurdum:
Proposition A = the absolute truth does not exist.
Proposition B = proposition A is absolutely true.
If proposition A is true, then proposition B is false. If proposition B is true, then proposition A is false. Regardless of the case, it is evident that at least one absolute truth exists. Therefore, an immaterial reality that trascends space and time who is intelligent and capable of thinking such truth must exist. Therefore, God exists.
The argument from YOU
This one is not of my authorship, it is brilliant. The atheist vision of the universe is materialistic, it states that only what can be perceived with the senses truly exists.
Proposition 1 = if materialism is false, then God exists.
Proposition 2 = if I can find at least ONE thing that is fundamentally immaterial, then materialism is false.
Proposition 3 = YOU exist.
Proposition 4 = YOU cannot be identified with anything strictly in your physical body. If that was the case, as the atomic composition of our body changes every ten or so years, any criminal brought to justice after ten or more years could get away by saying “I didn’t do it, it was another group of atoms and molecules who did it”. But the judge would say :”but YOU isn’t something constrained to your material composition, therefore your argument is false”.
Conclusion 1 = something fundamentally immaterial exists: YOU! (from propositions 3 and 4).
Conclusion 2 = therefore materialism is false (from proposition 2 and conclusion 1).
Conclusion 3 = therefore God exists (from proposition 1 and Conclusion 2).
This last argument, along with four more can be found in this video.