how did Islam affect early western civilization?

Right from the start of the birth of the new religion of Islam in 622, the Arabs that espoused it were perceived as being bent upon destroying Christianity and thus Western civilization most definitely took shape as a reaction against Islam.

By 638 the Arabs had invaded and conquered Jerusalem - creating much angst for the Christians as well as the Jews - and when the Arabs headed for Europe via Spain, there was real concern that the whole of Europe would fall under Arab rule and Christianity would die out in Europe.

The Pope in Rome was in peril. Not even the island nation of England was seen as being a safe haven for the Christian church - and Roman power - and that is why Rome chose far-away Ireland to be the last bastion of Christianity.

Actually, the Arab conquerors - unlike their Christian counterparts - were far more interested in acquiring wealth and power than converts. Wherever they conquered, Jews and Christians were heavily taxed and treated like second class citizens - like all subject peoples are, whoever the conquerors - but the Arabs did not force subject peoples to convert to Islam. Until Ferdinand and Isabella expelled the Arabs at the end of the 15th century, the Arab kingdom of Cordova in Spain lasted for almost eight centuries and was highly civilized.

It was therefore not so much Christianity that was in peril by the advancing Moslem Arabs but Roman power and wealth.