Is it permissible for a Muslim to boast that Allah is with him and answers his prayers, and to mention this to others to demonstrate the virtue of reliance on God and piety?

There is no objection to a servant speaking of God's blessings upon him and the answering of his prayers, and explaining the benefit of adhering to acts of obedience and relying on God, provided this is supported by Sharia evidence, encouraging good deeds, and recounting stories of answered prayers. This should be done from the perspective of inviting people to God and encouraging them to connect with Him, not out of blameworthy boasting, especially if the servant is not grateful for the blessings, in accordance with His (the Almighty’s) saying: "Indeed, Allah does not like every self-deluded boaster." The self-deluded boaster is one who sees himself better than others, is conceited, arrogant, and boasts about what God has given him while being ungrateful.