My question then is why does God/Jesus need to force us to make that choice, it seems totally unnecessary & cruel. And what happens to people who believe in other Gods? (like it says in the 1st commandments)
Again, I am not saying this is what I personally believe, I haven't decided on that, but the Christian theory is that all the other Gods won't show up as they aren't real. I'm not saying that it right and fair, just explaining the theology of people who take it literally.
So the only ones who show up are Jesus and the Anti-Christ. And loads of people will love the Anti-Christ as he is a deceptive bugger who will promise all things to everyone and people will be swayed by his promises of instant riches and really material, transient stuff. So Satan will be far more tempting for a load of people than Jesus stood there preaching kindness, but not promising to pay their mortgage off. So it comes down to people thinking about the greater good and choosing that, ultimately.
As to why this is necessary in the first place: I shall tell you the thinking about it but don't blame me if it sounds mad or it bores you shitless 
Ok, when Satan was in Heaven with God, when the Earth was made, he was really close to God. Then Satan thought it was unfair that he could only act on the powers and authority of God, and started to get arsey about it. They had loads of arguments and Satan kept pushing it and wouldn't accept that God was in charge by virtue of being, well, God.
So, Satan wouldn't do what God asked and was being a pretty rubbish angel, and he wouldn't be persuaded otherwise by God. So, God had to kick him out.
Satan came down to Earth and started acting in the opposite way to God. They kept arguing. In the end a deal was made. Satan knew that God doesn't control the actions of people on Earth. Otherwise they would just be like Gods personal playmobile people and what would be the point of them? So, he gave us free will, and had enough faith in us that we would follow him anyway.
Satan decided that he was FAR more appealing than God, and that people would want to follow him. So God said "fine, let's see what happens, if they follow you then you are in charge and if they follow me then I am".
God used to speak directly to humans (like Moses etc) but over time we turned away. God was frustrated because he could see us winding up with Satan who by this point was really rather a cunt.
In those days under Jewish law, people atoned for their sins through sacrificing animals. The bigger the animals, the bigger the sacrifice abd the absolution. All these being flogged in the Temple for vast amounts of cash, so the poor could never hope to atone like rich people could.
So, God sends Jesus to say "hang on, this is corrupt, everyone has access to God, not just the rich, who have been corrupted by greed at the hands of Satan".
God made a deal with Satan. That Instead of having all the souls of the people on the Earth who followed Satan, Satan could have his son instead, but he couldn't have The souls of anyone who ever believes that Jesus is the Son of God.
The souls of people who had followed Satan were separated from God permanently when they died. Satan thought about it, decided it was the ultimate two fingers up to God, and said "yeah, I'll trade all the human souls for your Son". So, Jesus gets crucified and is separated from God (hence "My God why have you forsaken me").
BUT what God knows and Satan doesn't, is that God has made Jesus special, so that he can actually come back from being dead. But Satan wasn't aware of that when he made the deal.
So Jesus comes back, Satan is truly pissed off as he has agreed that he will let go all of the people who believe that Jesus is the Son of God. People believe Jesus is real due to being resurrected, and anyone who believes gets to go to Heaven.
SO, Satan is still pissed off and trying to get people to not believe in Jesus, as part of his monumental hissy fit with God, but as per the deal, it's either God or Satan that get people. Hence people having to choose.
Apologies for the length, and I have probably got loads of that wrong, but it was how it was explained to me.