No such thing as a former Christian
Once you are saved you are always saved; you cannot be lost. God predestined or predestination some to heaven and some to hell. Is this as a messenger? For certainly a person can lose their salvation if there is such a thing as salvation. But I am a Torah Observant Jew who believes that Yeshua is the Messiah. I am of the Chosen People, Am Yisrael, and chosen in Him in Christ. A former Christian may be a born again Jew who is not a Trinitarian Christian or not a Christian in name only or one who does not obey or observe the commandments. A professing Christian most certainly is a Christian, should we not doubt his salvation, but there were believers in Yeshua as the Christ, Jews (the early church) and Gentiles before there were Christians, if the word Christ, Messiah, Moshiach, Mashiach, the Anointed one, was used.