The short answer means it is someone who is convinced to be an Islamic terrorist. We tend to see it as someone in a foreign land that has come under the influence by the terrorist. It also has the meaning of those in this country who may have grown up here and yet have fallen prey to the false teaching of those who practice hate with the end result being murder of innocent people. It is this second kind which tends to wreak havoc in our own land. It would be a valuable tool to invest time and learn about Islam. There are good books and articles that have been written by our brethren warning of the doctrine of Islam.
There is a pressing need for all of us to do a better job in our local congregations of instilling faith in our youth before they leave home. The beginning of faith in Jesus is much like a baby in need of milk (I Peter 2:2). The Bible leaves no doubt that such a beginning must progress to a more mature faith in the Lord (2 Peter 1:5). In the parable of the sower, the one heart which was successful was the one which in an honest and good heart, hear the word, keep it and bring forth fruit (Luke 8:15). There is a need with each generation growing up in the church to establish the basics of faith in Christ. Elders need on a regular basis to meet with teachers and discuss how the children are progressing in their classes.
Whether one ever learns about Islam would have no bearing on salvation. The child that is rooted and grounded in the truth, leaves home and can with the knowledge they have been given, refute any error. We do not preach hate nor do we condone error but do all in our power to lead men to Christ (Eph. 4:15). Our battleground may very well be in our own congregations where the Word is no longer viewed as the standard. Children growing up under that kind of teaching (lack there of), will make them susceptible to any and all false doctrines. Let us preach the Word and lead men to Christ, the real answer for life and eternity.