Ajani's Pridemate has always been an extremely powerful card without a true home in Standard. Heliod, Sun-Crowned and Daxos, Blessed by the Sun from Theros Beyond Death give mono-white lifegain the push it needs to become a powerful enough deck.
Mono White Devotion Lifegain Decklist
4 Faerie Guidemother/Gift of the Fae 1 Giant Killer/Chop Down 4 Alseid of Life's Bounty 4 Ajani's Pridemate 4 Daxos, Blessed by the Sun 4 Heliod, Sun-Crowned 4 Linden, the Steadfast Queen 3 Ajani, Strength of the Pride 4 Conclave Tribunal 2 Dawn of Hope 4 Healer's Hawk 3 Castle Ardenvale 20 Plains (331)
Arena-Formatted Decklist - Mono White Lifegain 4 Alseid of Life's Bounty (THB) 1
20 Plains (ANA) 61
4 Ajani's Pridemate (M19) 5
4 Daxos, Blessed by the Sun (THB) 9
4 Heliod, Sun-Crowned (THB) 18
4 Linden, the Steadfast Queen (ELD) 20
3 Ajani, Strength of the Pride (M20) 2
4 Conclave Tribunal (GRN) 6
2 Dawn of Hope (GRN) 8
4 Healer's Hawk (GRN) 14
3 Castle Ardenvale (ELD) 238
4 Faerie Guidemother (ELD) 11
4 Alseid of Life's Bounty (THB) 1
Heliod, Sun-Crowned complements Ajani's Pridemate and lets us go all in on the effect. Heliod is an incredibly busted new card that is sure to see combo play in other formats (such as with Walking Ballista), but in Standard this effect is extremely powerful when played "fair" (though some games it certainly won't feel very fair for your opponent).
Linden, the Steadfast Queen is a card from Throne of Eldraine that did not see any play until now, but it happens to do everything we want. She's an efficient creature in her own right, but she also provides triple devotion for Daxos and Heliod while also giving us a ridiculous number of individual lifegain triggers.
Alseid of Life's Bounty is a perfect fit for this deck, as we crave 1-drops that have lifelink and it can protect our Ajani's Pridemate from hard removal or just make it unblockable. Healer's Hawk is the classic curve out with Pridemate, as it is an evasive source of lifegain that can likely connect every turn. Giant Killer is our current choice for last 1-drop in the deck due to its power level, but it is worth considering cards like Beloved Princess and Law-Rune Enforcer.
Faerie Guidemother doesn't have lifelink on its own, but it is incredible here. It's nice to have a few more evasive threats to get in damage and trigger Linden easier, but the adventure half is amazing on Ajani's Pridemate. Similar to the protection from Alseid, the evasion can let our massive Pridemate or Heliod-buffed creature break through a stalled board.
Daxos, Blessed by the Sun is another awesome new card. Since we're playing mono white, it's basically never going to die to damage. It provides a ton of life gain triggers, but since it's legendary and is unlikely to die we only want to include three copies.
Ajani, Strength of the Pride and Dawn of Hope are here in small numbers to give us a powerful late game plan if our initial aggression is dealt with. Dawn of Hope easily lets us convert all of our mana into cards each turn, and Ajani gives us more copies of Ajani's Pridemate. The ultimate on Ajani is a real threat against non-aggressive decks since we're very likely to be at an incredibly high life total.
Finally to round out the deck we have Conclave Tribunal. It's worth noting that we now have access to Banishing Light, but since we have so many random 1-drops already Conclave Tribunal has more potential for a powerful swing turn. A notable absence from the convoke slots of the deck is Venerated Loxodon. In our playtest games we were finding that with all of our lifegain synergies, we never wanted to take a turn off to tap out for it. However, it is still at the top of the list for powerful white cards in Standard, so as we further tweak the deck it is likely that we will come to the conclusion that its exclusion is a mistake.