The 2 biblical contexts for “pray without ceasing”
The first context is 1 Thessalonians 5:17 — Spiritual Formation
The apostle Paul says, “pray without ceasing,” (1 Thessalonians 5:17). But does he mean that we must pray [literally] all the time? Not at all…. What Paul meant by “praying without ceasing” is to encourage the Thessalonians [as young, growing believers] to maintain a faithful prayer life as his own. Praying without ceasing does not mean to pray constantly; but to be consistent and persistent in our prayer life…. (Doctrina et Defensus, brackets mine)
Without consistent and persistent prayer, there can be no proper spiritual formation for the Christian. If one “folds” or gives up in prayer, he/she has given in to a great detriment. As Hebrews states in chapter 6, verses 10 and following, the Christian must show diligence in believing, so that with faith and patience he/she can obtain the promises God has given us for becoming mature as Christians (i.e. like Christ Jesus). We become like Him through partaking of His divine nature, which allows us to escape the corruption that is in us and in the world due to evil desires (2 Peter 1; Eph. 4:13). The more consistently and persistently we pray, the more we are illumined to the Scriptures and the more we access Him for spiritual growth.
The second context for “pray without ceasing” is Ephesians 6:18 — Spiritual Warfare
In perhaps the greatest revelation of spiritual warfare in the Bible, the Christian is warned to be consistent and persistent in prayer. This is very much like a warning 1 Peter 5:8 which states, “be clear thinking, be on alert, because your adversary the devil walks around like a roaring lion, seeking whom he may devour.” But whereas 1 Peter 5:8 commands to have an eye out at all times for the enemy, Ephesians 6:18 tells believers to keep their eyes intently on God, from Whom our help comes. We should do this for one another and for ourselves. Amid spiritual warring, when we seek justice against our spiritual adversary, the Christian ought not lose heart in consistently and persistently praying (Luke 18:1-8). If a judge who is unjust gives an otherwise helpless but persistent widow her victory, then how much more will the Righteous Judge give those who are his children the victory over their adversary? But, pray, pray, pray.