David is the first instance in which there is a genuine blending of the Christian faith and the styles and forms of Classical figural art. Here is the first time that biblical figures are represented in the styles of the Classical masters. Yes, he is not the freestanding nudes we recognize- the Boxer, the Discus Thrower, those trim and muscled figures. David clings to the vestiges of Gothic figures, but he is nude and freestanding and has paved the way for more of his kind. 

Donatello had a clear interest in the human form. Many believed when he unveiled the piece that it had to have been done in life, it was so true to form. He also, though, was the first of his kind to use bronze in a freestanding nude (this hasn't been done since the Classical era). He is finally making a statement of Italian society, and the importance of blending Christian art and modernizing life.

This is David, also sculpted by artists like Donatello and Michelangelo. The head of Goliath sits at David's feet. John the Baptist was also beheaded, but by Salome, a woman. Hercules and Samson tend to be represented with more muscularity, and Moses is usually an older figure. 

Work is in the public domain, accessed through WikiArt: http://www.wikiart.org/en/andrea-del-verrocchio/david-1475

"Contrappusto" (meaning counterpose in Italian) is the term used to describe a posture in which the figure's weight is shifted to one foot, causing the shoulders and hips to twist off the body's central axis so they are no longer parallel, as seen in David. It was introduced in Classical Greek art and rediscovered in the Renaissance.

This work is Donatello's David. It predates the Davids of both Michelangelo as well as Bernini. Donatello was known among his colleagues for his freestanding work, and had in fact created a freestanding marble version of David that predates even this one. That David, though, was not nude where this one clearly is.

This sculpture is a depiction of David, the young man who defeated the Giant Goliath in the First Testament story of the bible. He can be identified by his youthful appearance (David was one of the only youths being depicted at the time, apart from the baby Jesus). He can also be identified through his pose, where he stands atop the severed head of the giant Goliath, as the victor in their battle. 

As is traditional, Donatello's statue is made of bronze, after the practice of Classical Greece. Though marble is a more popular medium later (it is cheaper and its whiteness appealed to the Renaissance masters), the freestanding bronze works are a testament to the renewed interest in Classical works.

Sculptures by Donatello (ca. 1386–1466) often feature a spiraling "figura serpentinata" composition style that entices the viewer to move around the piece to view it from every angle. Sculptors who came after him often used this technique, and it could be said that he introduced it. 

Michelangelo's Pietà follows the traditional theme of such pieces of art by having the Virgin Mary cradling a dead Jesus Christ after his crucifixion. Michelangelo depicted Mary in stunning realism for the sixteenth century, while also presenting her as extremely youthful. Michelangelo claimed that a completely chaste woman would hardly age, despite being the mother of a thirty-three year old man.

Given the nature of the work (David is nude and sensually posed), the patron was unlikely to be the Church. Though it was not wholly unusual later to see the church represent its prominent figures in the nude or seminude, this particular piece predates that. Donatello also would not have had the resources to commission this work himself, and David is a biblical figure. It is believed, actually, that the Medici family commissioned the piece for their courtyard.