You have only one purpose in life. Once you know that purpose, you are to align your life and make choices consistent with it so you are on your purpose, rather than off your purpose. That's the essential message of being on-purpose.
In business, and especially in the church community, many people write about their "purposes." Note the use of the plural form, a recent pop culture invention mostly due to Pastor Rick Warren's book The Purpose Driven Life in which he tells people they have five purposes of Worship, Fellowship, Discipleship, Ministry, and Evangelism. These are missions of the church, not purposes. For all the good Pastor Warren's book is doing, it is ultimately confusing people at the core of their being, but it is getting them engaged in life more profoundly.
You do not have many purposes in life. You may have multiple visions and missions, but only one purpose. You may have many reasons for doing something, but only one will to do it. (Read more about the subtle differences between purpose, vision, mission, and values by clicking on this sentence.)
Need further proof of the singular nature of purpose? The Merriam-Webster Online Dictionary doesn't provide a plural for purpose. Purpose is inherently singular.
For example, here is an excerpt from a devotional I read today, "God
will provide the necessary circumstances to accomplish His purposes in
your life." This is incorrect because of the plural use of the word. God has one purpose for your life, and, yes, he will arrange the circumstances. To push the point just a bit further, let's replace purposes with the word will. This is a more accurate description of the usage. God has one will, not a plurality of wills.
Reason and purpose are frequently interchanged. Life purpose is a person's one reason for being or raison d'etre. Purpose in lieu of reason is an appropriate, but a less effective word choice. Purpose is deeply about one's intention or will. I appreciate that one can have many reasons for doing something, but there is one intention. To be otherwise is to be double-minded or confused. That is the very challenge purpose clarifies for you. You can't have purposes or be on-purposes. The logic and semantics don't support the concept of purpose – plural.
Avoid getting caught up in this sloppiness of use. Strike the non-word purposes from your vocabulary. Purpose is a truly special and remarkably powerful word as applied to one's life purpose. Seek the singleness of strength of being of one mind, body, and spirit and you will be on-purpose.
Your comments are always invited.