"Love everyone with a strenuous love based on charity, but form friendships only with those who can share virtuous things with you." - St. Francis de Sales
The below is taken from The Introduction to the Devout Life by St. Francis de Sales:
1. The greater the virtues you share and exchange with others, the more perfect your friendship will be.
If this participation is in matters of knowledge, the resulting friendship is certainly praiseworthy. It is still more so if you have virtues in common, namely prudence, temperance, fortitude and justice.
2. If your exchanges are based on charity, devotion and Christian perfection, O God, how precious this friendship should be!
It will be excellent because it comes from God, excellent because it leads to God, excellent because its bond will endure eternally in God. I do not speak here of simple love of charity we must have for all, but to that spiritual friendship by which two, three or more souls share with one another their devotion and spiritual affections and establish a single spirit among themselves.
"How good and pleasant it is for brothers to dwell together in unity." - Psalm 132:1
3. Embrace true friendship by encouraging one another to perform good deeds. For those who live in the world and desire to embrace true virtue, it is necessary to unite together in holy, sacred friendship. This way they assist and encourage one another to perform good deeds.
4. It is necessary to find those who will support us and need our support in promoting the devout life.
This is holy partiality that separates good from evil, the sheep from the goats, the bees from the hornets. It is a distinction that has to be made.
5. Sacred friendship uses plain, sincere language and it praises nothing but virtue and God's grace, the only foundation on which friendships rests.
False friendship brings on dizziness of mind which causes people to falter in chastity and devotion and carries them on to improper looks, sensual caresses, deep sighs, little complaints about not being loved, enticing postures, acts of gallantry, requests for kisses and other familiarities.
6. Holy friendship has eyes that are clear, gestures that are open, and familiarities that are entirely spiritual, complaints only that God is not loved - all infallible marks of purity.
Worldly friendship impairs the judgement and leads to rationalization. False friendship, even when it is repulsed, leads to slander, trickery, sadness and jealousy.
7. Friendship is at all times and in all ways honest, courteous and amiable. It never changes except into a purer and more perfect union of minds, a living image of the blessed friendship that exists in Heaven.
8. Friends love each other despite their faults and flaws. Just as a gold miner separates the precious metal from the earth or sand, so friends must sort for the best in each other and treasure it.
St. Gregory Nazianen testifies that many people so admired St. Basil that they even imitated the style of his beard and his mannerisms of speech and gesture. Husbands and wives often acquire each other's mannerisms. Friendship requires that we assist one another in freeing each other from imperfections. We must, of course, meekly put up with a friend's faults, but we must not lead him into our faults, must less should we acquire his.
But I speak only of imperfections. As to sins, we must neither occasion them nor tolerate them in our friends. What a strange or sinful friendship that would watch the friend die spiritually!