Tulsidas was a great Saint. He was a devotee of Shriram. Before He became a Saint, He performed regular spiritual practice for many years as a seeker. During that period, He used to feel that Lord Rama is superior to all other deities and would not even glance at another deity's image.
Once Tulsidas went to Dwarka (a city in modern day state of Gujrat, India, which is a birthplace of Shri Krushna to visit Nandadas, a devotee of Shri Krushna. On reaching there, he found Nandadas in the Krushna temple. Tulsidas could not bear to look at the idol of Shri Krushna. He could not understand why Nandadas worshipped Shri Krushna instead of Lord Rama.
Nandadas was saddened by Tulsidas' incorrect attitude towards other dieties. He prayed to Lord Krushna to help his friend. Within minutes, the idol of Krushna was transformed into that of Lord Rama. Immediately, Tulsidas realised His mistake. He realised that both were different forms of the same Lord and none was superior to the other. He felt repentant and tears started rolling down His cheeks. He hugged the idol and asked the Lord to forgive Him. Nandadas was happy too, as Tulsidas had finally realised the Truth. Both felt gratitude towards the Lord.
Moral: This story implies that all the Deities are different forms of the same Lord, but we see them as different due to our (spiritual) ignorance. To realise this truth, one needs to undertake regular spiritual practice.