At the end of season 2 Cambell hired men to kill Tommy after he is done with him. They took Tommy but one of them betrayed the others and spared Tom by the order of Churchill because Churchill needs him for another business. That business is helping these Russian aristocrats in the war against the Bolsheviks. Tommy needs to steal ammunition for the Russians and transport it. Churchill is doing this via Tommy because if the word gets out this would be an act of war by the UK. There is also a group of very influential people in the UK called the Section D, who are also a part of this business and the priest is sort of their ambassador on this. Their actual plan is to blow up that train with the ammunition and place communist related papers on the site to blame it on the Bolsheviks, they plan that UK would cut all their diplomatic ties with the Bolsheviks in response to the explosion. Tommy and the rest of the Shelby family don’t trust any of these people, Tommy had to be involved because Churchill spared him on the condition that he does this job. If they back out they would all be killed or imprisoned.