Single and first meeting is likely to be deceptive. People pretend what they are not. However single meeting is never enough to know about character. One needs a long association with that person instead of single meeting to judge his credential of character.
One Swallow does not make a summer
The quotation is to interpret symbolically.
The swallow is a migratory bird which during extreme winter migrates from cold region in the north to tropical region. If one bird comes to the tropics it does not mean summer has come.
When swallows in cluster come that marks the advent of summer.
In single meeting maybe a person behave saintly appearance to dodges others.
Similarly there may be a person who donates large sum of money to charitable institution but he may have done it to get benefit from tax concessions.
The money he donates may be a part of black money but he seems harmless and selfless. People can take rich men as saints.
Though his motives could be of fully worldly nature.
A single good act is not enough to know real intentions of people. It is not easy to realize motives of people. Similarly a person needs to read many books before he reaches a conclusion.
Reading of a single book on a particular subject is not sufficient to gain a complete understanding of it.
Researchers have to lay their hands on past research paper to find out of a new way out of problem.
They have to read extensively to arrive at a conclusion.
Hurried assessment can be faulty or partial.
One should develop critical outlook to reach the depth of things.
Just as the arrival of swallow does not indicate the advent of summer, likewise partial analysis of things cannot help one to arrive at a correct conclusion.