The message of the exclusiveness of Christianity as the only way to God is not something contrived by Christians, but was taught plainly by our Lord himself.
Matt 7:13 “Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. 14 But small is the gate and narrow the road that leads to life, and only a few find it.
Proverbs 14:12 There is a way that seems right to a man, but in the end it leads to death.
John 14:6 Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through me.'
John 10:7 Therefore Jesus said again, “I tell you the truth, I am the gate for the sheep. 8 All who ever came before me were thieves and robbers, but the sheep did not listen to them. 9 I am the gate; whoever enters through me will be saved. He will come in and go out, and find pasture. 10 The thief comes only to steal and kill and destroy; I have come that they may have life, and have it to the full.'
1 John 5:10 Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has not believed the testimony God has given about his Son. 11 And this is the testimony: God has given us eternal life, and this life is in his Son. 12 He who has the Son has life; he who does not have the Son of God does not have life.