Can someone who has come to believe in Jesus Christ lose their salvation again?
Answer: Faith in Jesus Christ as the son of God, the saviour and the healer of mankind, is at the same time both a gift of Gods generosity and, on the part of a person, the outcome of the freely accepted gift bound with conversion from sin and turning to God. God respects freedom of will. He expects full personal responsibility in giving. Thus he neither forces us to accept faith nor does he automatically grant it to us. It is the nature of true faith that it is freely given and freely accepted.
This is already really the answer to the question. A person can neglect or even reject faith, even after it has been freely accepted. Whoever, of their own free will, neglects or even consciously gives up faith in Jesus Christ as the Son of God, in doing so rejects also the gift of salvation that was offered to them and which was at first freely accepted. Such a person chooses distance from God or even consciously defies God. Hell means to close oneself off from Gods love for ever and thus to have put oneself in the absolute calamity of distance from God, yes, even enmity towards God, which is hell.
Neither in Holy Scripture nor in the Church's teaching tradition is it categorically said of any particular person that they were truly in hell. Rather, hell is always kept in the forefront as a real possibility combined with the offer of conversion and life.