'A' emphasises the direct connection between 'the matter' and 'your fate'. What is important is that the 'matter' relates to 'your fate'. 'Your fate' is going to be very important in the conversation that follows.
In B, although the 'matter' is 'related' to 'your fate', there is less sense of importance (or urgency) in the conne
Thank you, MrPedantic. But what about "The matter that relates to your fate cannot be taken for granted"? Is it right to say "The matter relating to your fate cannot be taken for granted"?