"Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I am the first, and I am the last; and beside me there is no God." Isaiah 44:6
If you have been saved by the grace of God and if you have met the Savior; if you have put your faith in the person of Jesus Christ, it is because the Holy Spirit has come to reside in your heart. You know there is only God in the person of Father, Son and Holy Spirit. You do not need to try to convince others to be faithful to God, but you do have to bear witness that Jesus is the only Son of God and that God is Father, Son and Holy Spirit.
Other religions have different gods. That's OK. God knows all about it. God is not in a fret because many people call on other gods. Do you have it right? Do you know the God of Israel? Do you place 100% of your faith in Jesus? When you get this right, you can stop worrying about all those who believe differently. Our God is about a personal relationship between you and God, period. Of course we need missionaries and of course they and you are to tell the world about Jesus. But the right way to evangelize is to speak from personal experience about your relationship with God and not to deface or otherwise condemn others who believe differently. If you tell someone about your relationship with God, the Holy Spirit will take it from there.
Foolish and unnecessary arguments with those who believe differently is not the way. Personal testimony is. God gives personal testimony of Himself in this verse of scripture. Maybe we should do the same.