Messi wasn't singlehandedly carrying but he had the most impact on the pitch for Barcelona. We also cannot deny the impact players like Xavi, Iniesta or Neymar had but Messi had the most impact. Before Neymar and Suarez joined, he had the duty of scoring goals and he did that very well. Even scoring 91 goals in a calendar year.
(I dunno how he did that) After Neymar and Suarez joined, he moved to the right flank and assisted them with tons of goals also while he scored lots by himself.
In my opinion, this is a biased statement - Xavi and Iniesta made the greatest contribution to the team. If you remember Xavi and Iniesta won the 2008 European Championship, then the 2010 World Championship and then again the 2012 European Championship - the greatest achievement. They did it without Messi, and without them, Messi himself failed at the 2010 World Cup and then at the 2011 America's Cup.
At everytime Barcelona always has great players.
Players like Eto, Ronaldinho, Ibrahimovic, Ture Yaya, Xavi, Iniesta, Messi, Neymar, Saurez etc.
These players have their best and played their quota in making the club great.
Yet, there impacts divers based on the time they arrived, their wings and loyalty.
Xavi and I Iniesta had great impact because they span to different generations and were doing it when it seems no one else could handle the midfield.
Messi having rose to arguably the best player in the world, still remained with Barcelona in what seems like a life contract.
So, their levels of loyalty differed according to their roles and how the club made them. I think Messi was Barca made and he gave back to Barca.
On one occasion, Iniesta did say that CR7 was a very good player, even for him he is the best in the world, because everyone talks about Messi, and they have never drawn the conclusions that Messi only pushed the ball into the opponent's net and yes, that's it what it is about, but the work of Xavi and him was never valued, they only saw Messi, on the contrary if instead of Messi there would have been CR7, Barcelona would have had many more trophies, because CR7 on the pitch playing like in life really is a great person.