The whole of chapter 13 (Deut 13:8-9) speaks in these term of putting to death anyone who tries to turn the Jews away from the true living God to follow false gods.
Firstly let us look at this from a view point of the nation of Israel. This judgement may seem harsh to us today but let us dig deeper. The nation of Israel was told to follow the true God only and then they would be blessed. They were also told that if they followed false gods they would be cursed. What this meant was that if the Jews were to turn a blind eye to this sin, then it would spread like gangrene through Israel which in the end would bring even worse consequences for the whole nation. This is exactly what happened as we see as we read through the old Testament. Because Israel failed to deal with this issue as soon as it appeared, the whole nation fell into the worship of idols which in the end (after much warning from the prophets that God sent) led to very stern judgement from God. If only the Jews had heeded God’s warning right back at the beginning.
Looking at this from the Christian viewpoint, we Christians have a different covenant with God. We are not told that if we obey all his commands we will be blessed and if we disobey we will be judged. At least not as a nation – we as individuals can know the blessing of God but that is not in the physical sense as the nation of Israel receives. We may know blessings in our lives from God while in this life but we will receive our full reward when Jesus comes back for us - “Serve wholeheartedly, as if you were serving the Lord, not people, because you know that the Lord will reward each one for whatever good they do, whether they are slave or free. (Eph 6:7-8)” In the same way God may judge people or nations of the world in this life but the final judgement is reserved for the world at the return of our Lord. God deals with Israel as his chosen people in a different unique way at this time and in the past.
Deedat’s real reason for bringing this up is to try and defend the Islamic practice of killing apostates. This is a common defence method in Islam when trying to defend the behaviour of its prophet. That is to look for examples of similar behaviour in history or the Bible and use that as evidence that such practices are fine with God. The fact remains that to date I have never heard of a genuine Christian trying to kill someone for leaving the Christian faith. We are free to come and we are free to leave. We may be sad when someone rejects Jesus as a saviour but we do not have any desire whatsoever to do them harm. The standard Christian response is to pray for them.