For so long as I can remember hearing the term, devotional has been a synonym for "religion lite." It refers to something that touches the heart and makes one think about God and being a better person. Many "devotionals" are sugary and full of cotton candy. But when one reads a few of the places in the Bible where devote, devout and devoted are used, a much stronger concept emerges.
Jericho was devoted to God which meant it was to be utterly destroyed and all within belonged to God (Josh 6:17, 19, 21). Achan was himself utterly destroyed for taking from the devoted things. Anything devoted to God was holy to him and for his service or for the priests (Lev 27:28, Num 18:14). Should a city in Israel be proven to be idolatrous then it was devoted to God, that is, utterly destroyed, all inhabitants killed and the city never rebuilt (Deut 13:12-18, Lev 27:29).
Although the N.T. does not impose such temporal penalties, the words imply such dedication as to be described as fanatical. For the feast of Pentecost, "devout men from every nation under heaven" gathered to hear the first gospel sermon. These were so fanatical that they traveled for days or weeks at great expense to spend a few days in the holy city. Cornelius was so extreme that he deserted the religion of his family and world to do good for a despised people, the Jews, and converted some of his soldiers (Acts 10:2-7). Further, his devotion influenced his family and friends to be there to hear Peter 10:24).
Jesus demands the utmost devotion, an all or nothing zeal, "You cannot serve God and Mammon" Mt 6:24). "No man, having put his hand to the plow, and looking back, is fit for the kingdom of God (Lk 9:62). Paul exemplified such devotion, "It is no more I that lives, but Christ lives in me (Gal 2:20). Every Christian is required to devote himself in this absolute way, "Put to death therefore your members which are upon the earth;" "Therefore" refers to, "If then you were raised together with Christ" (Col 3:5, 1). The Holy Spirit leaves no room for sometime or somewhat religion: Kill yourself or lose all hope.
Devotion begins with the mind, "Finally, brethren, whatsoever things are true, whatsoever things are honorable, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things." A Christian is not even allowed to think what he wishes. His heart belongs to Jesus. But further devotion comes with Paul's next exhortation, "The things which ye both learned and received and heard and saw in me, these things do: and the God of peace shall be with you (Phil 4:8-9). We are not free to do as we wish, not if we are to be described as devoted. Nearly every epistle makes the same statements using various figures from slavery to "Walk as he walked" to "Present your bodies a living sacrifice" and more (1Jn 2:6, Rom 12:1).
Given these and many similar passages, how did devotional ever come to mean something "feel good"? Many of the sayings of Jesus and of the N.T. are far from nice. They are demanding in a "get busy rowing or get out of the boat" way. A devotional should call on Christians to give their all to be holy in the service of God.
Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him. For all that is in the world, the lust of the flesh and the lust of the eyes and the vainglory of life, is not of the Father, but is of the world. And the world passes away, and the lust thereof: but he that does the will of God abides forever (1John 2:15-17).