If a Christian "converts" to Judaism or Islam (or any of the Abrahamic religions), is he still considered saved as a Christian? What if he/she converted from Trinitarian to Uniterian? This is a general Christian question.
There is no consensus among Christians on this question. Assuming that the conversion is genuine and permanent, rather than momentary weakness, the three main schools of thought that apply to this issue are:
According to this view, Christians can lose their salvation. Thus, a Christian who converted to another religion would be seen to have lost his salvation, if he had it in the first place. Christians who hold this view, such as Arminians, would answer "No".
Perseverance of the Saints
Another school of thought is that Christians cannot lose their salvation, and that true believers will persevere till the end. According to this view, although true believers may fall and stumble temporarily, they will never fall away permanently. A supposed Christian who permanently converted would be seen as having never been a genuine Christian in the first place. Like the first view, Christians who hold this view would answer "No".
Free Grace Theology
Free Grace Theology is a view in which Christians cannot lose their salvation, and may fall away permanently. This view teaches that the saved may or may not become disciples, and may or may not undergo the process of sanctification. Christians may believe in Christ at one point, and subsequently forsake any identity with Christianity. Christians who hold this view would answer "Yes".
The Apostle John addresses this issue in his first epistle:
They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. 1 John 2:19 ESV
This teaches that those who leave [Christianity] were never really Christians.