[Sidenote: 1515] Was he already a Reformer?
Not in the later sense of the word, but he was a disciple of Erasmus.

Capito wrote to Bullinger in 1536: "While Luther was in the hermitage and had not yet emerged into the light, Zwingli and I took counsel how to cast down the pope.

For then our judgment was maturing under the influence of Erasmus's society and by reading good authors." Though Capito over-estimated the opposition of the young Swiss to the papacy, he was right in other respects.
Zwingli's enthusiasm for the prince of humanists, perfectly evident in his notes on St.Paul, stimulated him to visit the older scholar at Basle in the spring of 1516.

Their correspondence began at the same time.

Is it not notable that in _The Labyrinth_ the thread of Ariadne is not religion, but reason?
His religious ideal, as shown by his notes on St.Paul, was at this time the Erasmian one of an ethical, undogmatic faith.